Heavy

Heavy is the most consistent source of single-target DPS, good at whittling down crowds of tiny robots and exceptional at shredding Giants. Arguably the easiest class to play, there's usually no reason not to play Heavy, and an easy recipe for success is having a team with multiple Heavies.

Playing Heavy isn't exactly rocket science, but there are still some pitfalls that new players make as well as certain tricks such as bodyblocking that some players might ignore.

Common mistakes

Dumping everything into firing speed right away.

Given how quickly and easily you can get results by shooting robots at close range, it would make sense to make your already great DPS be even better. However, it's worth remembering that even if you don't upgrade your minigun's firing speed, the damage you do will be more than sufficient on the early waves. Your resources in the early game are limited, especially on Gear Grinder, and many new players make the mistake of not getting Health on Kill nor any resistances whatsoever in favor of hoping that a Medic or a Mad Milk will save them in a pinch (Expecting your team to cover for you: Bad idea.)

To repeat what should be obvious: If you're dead, you're not shooting the robots. Health on Kill and resistances are significantly more important in the early waves than being able to kill a giant robot 2-3 seconds faster. You should only fully dump into firing speed right away if you have a dedicated pocket Medic, which is not something you should expect unless you're queueing with a friend who'll do it for you.

Getting Knockback Rage.

This is a depressingly frequent occurence with new players. To cut to the important part right away: Knockback Rage reduces your damage by 50% while active. Not only that, but by pushing a giant away, you lose out on damage even further by not getting damage ramp-up, which you can only achieve by being right in a giant's face.

Though that alone should be enough to convince someone not to purchase this upgrade, many will argue that pushing away a giant could be a lifesaver in a dangerous situation. The obvious answer to this fallacy is that simply killing a giant is significantly faster than pushing him away, especially since you're playing Heavy! Furthermore, if knockback really was crucial (it isn't), then I would much sooner rely on a Pyro with a stock flamethrower (another depressingly frequent character) than a Heavy who could instead put the 350 credits into crit canteens for if a giant gets too difficult to deal with.

Furthermore, it's important to note that by pushing back giants, you make things harder not just for yourself, but for your teammates, too. If you're denying your fellow Heavies the chance to get full ramp-up on a giant, they won't be happy with you.

Standing on top of buildings.

A less frequent mistake but nonetheless something you see with complete newbies. There are two major problems with standing on buildings as opposed to staying on the bomb path: One, you lose out on damage ramp-up, which as discussed earlier is significant if you want the robots to be dealt with quickly, and two, robots will more often than not ignore you and walk past you while shooting at you, meaning that you are completely ignoring your role of being a roadblock for robots. On that note...

Not blocking the robots.

When Super Scouts or ubered giants start rolling in, the only thing that can stop them dead in their tracks is your body. Pay attention to the bomb path-- by correctly positioning yourself, you can outright prevent a giant robot from walking past you. This is most commonly used to stop Super Scouts, but you can also slow down an ubered giant by simply standing right in front of it, causing it to stop moving until it kills you. In this scenario, either hope that they're too distracted by shooting someone else, or that you have the Fists of Steel (and sufficient resistances) or an uber canteen (or a Medic) to keep you alive until the giant's uber runs out and your team is free to finish it off.

Following the tank.

Frustratingly common with new players. Heavy does only 25% of his regular damage to tanks. The only way to circumvent this crippling damage penalty is to equip the Warrior's Spirit and fully upgrade its attack speed, but dumping 800 credits into doing something that other classes are better at anyway is a stupid move (unless you're on the last wave). Just ignore the tank and focus on shooting down the robots. Thank you.

Upgrade priority

Largely dependent on what mission you're playing and what kinds of threats you have to face on the first wave. The short version is simply to keep an eye on what robots you'll face and to get resistances that counter the icons on the top of the screen. If you cannot afford many resistances, then you can spend 400 credits (the amount you start with on Gear Grinder) on getting two ticks of Health on Kill, which is significantly helpful for mowing down waves of tiny robots.

Following the first wave, you should balance out getting relevant resistances with getting firing speed on your minigun. Important to note that due to a bug, the second tick of firing speed does nothing, so account for that when budgeting your credits-- either get one tick of firing speed, or three or four. Not two.

After you have sufficient resistances and firing speed, you may optionally invest into movement speed and ammo capacity if you're the adventurous type that loves wandering away from the dispenser to get closer to the robots. In addition, if playing on Mannhattan, you can put 200 credits into two ticks of jump height, which allows you to swiftly get from the bottom path onto the platform on the left, by crouch-jumping onto the silver barrel next to where the dispenser usually goes.

Canteens should be bought sparingly and mostly saved for the later waves when you have money to burn. A crit or uber canteen can easily dispatch a giant robot, but you can hurt your upgrade budget by going overboard early on.

Weapons

Primary

Weapon	Rating	Review
Minigun	A	The default choice for Heavy is a good one, a great one even. A good amount of damage, and by having no penalty for movement speed or ammo consumption, you can be very flexible and quickly move around while firing, should you upgrade movement speed.
Brass Beast	S+	The highest DPS you can achieve as Heavy-- the best option and it's not even close. Seeing as the Brass Beast slows you to a crawl, one might think that the stock minigun would be better due to its superior movement, but once you've gotten enough experience you will no longer need stock's run-and-gun mobility as you will know exactly where to stand to do the most damage with as little downtime as possible, therefore making it obsolete and making the Brass Beast all the more appealing. Even if you don't know how to position yourself (likely scenario if you're a new player), the Brass Beast's superior damage will still do wonders for a dope that just sits by the dispenser and never moves. Simply put, there's no real reason not to use this thing.
Natascha	B*	The asterisk on the rating is there for one big reason: If there are no Super Scouts to deal with, then there is no reason to use the Natascha. The slowdown on hit will not affect other giants, and its effect on tiny robots will be miniscule-- two very big drawbacks on top of the Natascha's already painful damage penalty. Slowing down Super Scouts, as you might've guessed, is the sole upside of this weapon, and it does this job fairly decently. Sure, they might slip past you, but you'll slow them down and whittle their health enough that they'll be significantly easier to kill by the time they reach the hatch.
Tomislav	Bad	The Tomislav is a straight downgrade to the stock minigun. You might assume that the increased accuracy would help against robots that are far away, but there are two huge issues with this hypothesis: One, you should always be as close as you can reasonably get to the robots, and two, giants are such massive targets even at long range that the tighter spread of the Tomislav won't make any difference at all. There isn't any benefit to using the Tomislav that justifies its nigh-crippling 20% decrease in default DPS.
Huo-Long Heater	Bad	The fire ring itself does practically no damage at all, so the only upside to the Huo-Long Heater is its increased damage to burning targets. There's just a teensy-weensy problem with this upside: The Brass Beast still does more damage. Furthermore, because the Huo-Long Heater drains your ammo far quicker than the other primaries, you cannot justify it as being more flexible or mobile than the Brass Beast.

Secondary

Weapon	Rating	Review
Sandvich	Sandvich	It's the Sandvich. Just as in the rest of TF2, it's exceptionally helpful at keeping your teammates healthy, and you can patch yourself up if you suddenly find yourself at 20 HP after a long day (couple of seconds) of shooting a bunch of robots.
Buffalo Steak Sandvich	C	The Steak might seem like a handy tool at first; it boosts your movement speed for a while and buffs your Warrior's Spirit when you're punching the tank. However, its actual usefulness is disappointingly limited. The movement speed increase only comes into play if the teleporter is destroyed (which shouldn't usually happen if the Engineer knows how to count to five) and its synergy with the Warrior's Spirit versus the tank is questionable at best; It's much faster to just keep punching after the Steak's effects run out than to waste five seconds eating another one.
Dalokohs Bar	Bad	Having an extra 50 maximum HP just isn't a big deal in any stretch of the imagination. If you so badly need your maximum HP increased, then a Medic's overheal will do the job much better.
The Second Banana	Bad	The faster recharge of the Banana isn't really a boon because if you really need to be healing yourself from near death every couple of seconds, then you must be doing something wrong. Even if you have the Sandvich, your lunchbox shouldn't be your primary source of healing; that role goes to Health on Kill and the Mad Milk your Scout will gracefully (not) throw. More importantly, you're no longer able to fill up half of your teammates' health by throwing your lunch, so the more significant upside of having a lunchbox is now useless.
Shotguns	Bad	This goes for all three of the shotguns. There are only two reasons why one might equip a shotgun: either they forgot or don't have the Sandvich, or because they think they're a comedian who graduated at the top of their class at Clown University because they found out that Fat Scout exists. And before you ask, no, it's not viable in any capacity.

Melee

Weapon	Rating	Review
Fists of Steel	S	Resisting half or more of all incoming ranged damage is a big boon if you suddenly find yourself overwhelmed and needing to escape. What's more, because your maximum overheal is restricted while the Fists of Steel are drawn, a Medic latching onto you while you have them equipped will build up their ubercharge twice as fast. If you're using this tactic, remember that you only need to have the Fists of Steel pulled out at the moment the healing beam is attached to you; once you're connected, you can switch back to your primary and resume shooting as the Medic continues to build ubercharge.
Warrior's Spirit	B*	The asterisk is there to note that this weapon is really only useful against tanks. Running into a group of tiny robots or giants would be suicide, and the only other use would to knock out a Steel Gauntlet with a random crit (or a crit canteen if you like wasting credits). You shouldn't invest into this weapon until the later waves with tanks, as other classes will be much better at busting the tank than you.
Gloves of Running Urgently	C	The ability to both get to the front lines and run away from them quicker is a notable feature, but the drawback of the health drain is even more notable. If you drag yourself onto the front lines with only half your health, you better hope a Medic is there to compensate for that.
Killing Gloves of Boxing	C	The KGB have one specific use: getting a random crit on a nearby robot and using the short burst of crits to quickly switch to your minigun and do some extra damage. Sadly, this is not frequently practical unless robots are constantly running past you (which they shouldn't be!) and it's more convenient to simply purchase crit canteens instead.
Fists	Bad	What the hell are you doing, punching the robots? Come on now.
Eviction Notice	Bad	Worse version of the GRU, which isn't really amazing to begin with.
Holiday Punch	Bad	Robots can't laugh.