Heavy is my favourite class of all time, and is also the first one I played TF2. He is the one who attracted me to try out this game. I played him most of the time and my best records come from him. Even he moves the slowest in the game, I play heavy in assassin style, ambushing enemies and taking kills in close range. These tips are conclusions from 100+ hours of heavy playtime. Hope this guide can help players who are interested in playing heavy.
Tips:
-Always keep your minigun up in team fights. Team fights can be several waves of battles (e.g. 3-4 guys a time). I have seen heavies stop spinning their gun after clearing a wave of enemies, and get mowed down the next moment. It's a good habit to keep your gun ready when enemies will arrive soon. But don't do so at other times like travelling from base to front line. First off, this will make you walk even slower when you are slow enough already. Secondly, the noise will alert enemies of your position, which allow them to escape or ambush you.
-Find a corner when eating sandvich. Your back is very vulnerable, either from a backstab or from a backburner. Have your back stuck to a wall while eating your lovely sandvich can be a matter of life and death. At one time I encountered an enemy spy when I am eating sandvich, he tried to backstab me but failed, in fact he could have killed me with his revolver if he is smart enough, however, in most of the time spies will try their best to backstab an idling player, as this grant them a bonus in-game score. So learn from this story, and eat with your back stuck to a wall.
-Make yourself the center of a team fight, you are the toughtest guy in TF2 and you got the largest body. You are naturally the damage absorber of your team. You are especially good at doing your job with a medic, get them buffed you to 450HP before initiating a fight. Tell your team to spread out a little bit from you as explosives will be poured on you. Try to burst out as much damage as you could before dying. You are the badass, so die like a man.
My Builds:
Minigun + Sandvich + G.R.U.
This build is common among players, first off the standard minigun is the best one in my opinion, you can use another one, but the classic one won't fail you. e.g. Brass Beast is awesome when you are defending objectives, but don't expect you can ambush players successfully (it takes ages to spin up). Sandvich is a must, shotgun is not necessary when you have a 150 kg gun in your pocket, plus the fact that heavy can't heal himself without it.
The G.R.U. makes your life a lot easier when travelling from one point to another, but don't put it on during fights.
Tomislav + Sandvich + K.G.B
This build turns you into Rambo when talking about ambush your enemies. Take them by surprise with Tomislav (this is the hardest part, be patient and wait for enemies to walk past you), then take the killing blow using K.G.B. And if other enemies are around, finish them off with your gloves (195 damage per hit :D ).
Brass Beast + Sandvich + Fists Of Steel
This build is purely for defense. The reason of using brass beast for defense is pretty obvious. Only eat sandvich after surviving a wave of incoming enemies, don't eat it in the middle of a fight, this waste your precious firepower when your team need it the most. Equip fists of steel when you are out of ammo and is waiting for ammo box/dispenser to resupply. Crouch if there is no cover available, try to fire at enemies at mid-range to reduce damage drop. Ask your engineer to build and upgrade their dispenser first to supply you with ammo and health, turning you into a 'level 4 sentry'.