I've basically come full cicle in the past 18 months of playing TF2. I played Pyro until I got decent, then I played soldier until I got decent, then some sniper, so I wasn't crap, then back to my original class: spy (during the heavy update), and I moved into engineer, and then played a lot of scout, but I didn't like how it could be hard to take on the entire team, and you're efficetivness is really cut down to size if you have to run away all the time. I try some medic, but I haven't mastered the art of the ubercharge, and finally, I get into some heavy, and I'm really enjoying it.
Heavy is great for gaining ground, and for flanking. if you can put supressive fire on people, they're just going to get confused, and they won't run for cover fast enough. The shotgun is more powerful than I originally thought, and I've come to term with losing health during a fight (as a scout, I would only consider engaging if I had 100+hp). The heavy, of course, has a lot of way that you can circumvent his power. But I can handle most classes at the average-decent skill range.
Of course, being able to hog health packs is really useful as a heavy. If your team knows that you need them the most, then you can really survive wave after wave of enemies.
Scout can be a joke. If they don't get at poink blank to me, they can't get me. the sandman is a real problem, but I can dodge it often enough, that I'm not sick of it yet. If the enemy is playing a lot of scouts, I'll use natascha. It's more effective than sasha against scouts.
Playing against a Soldier, I try to get the high ground ahead of time if I can. But the soldier can't outshoot me. May be pro soldier can, but that's not gonna be a problem for me.
Pyros can't do much of anything against a heavy. They can charge me, to no avail, or they can try and flaregun me. If I am doing a good job of holding the health packs, then they can't really stop me until they go 2v1. Again, I'll use natascha against a team full of pyros. If I spot a pyro going after a teammate, slowing them down can be a real help when my teammate needs that extra second to turn around.
Other heavys I just avoid. It's too hard to determine who would win a minigun fight. I have yet to kill a heavy with the kgb, and work towards that acheievement. Anyone have anyextra suggestions for heavy v heavy?
Engineers themselves are easy, but the heavy is also a good sentry killer. He can both long range, and corner a sentry gun. He can take a level 3 sentry gun for a few seconds, so if you can plan a flank with the sentry gun, you can very well take out the engineer, and then come back for the gun after getting healed.
Medics are useful, but I think that people over exaggerate how useful a medic is. It's nice to heal the chips of damage that people cause, but the heavy is the one class that can be left along for a minute, and not die. He's more like a medic anchor than a medic hog.
Snipers, of course, are the bane of my existance. The problem is the heavy's speed thanks to the huntsman, I have to play 2 different strategies against 2 different weapons. A sniper in the back of the fight is easily my biggest worry. It force the heavy to get away from the action, and go into ambush mode. Sometimes, this is good, and sometimes, this is impossible.
Spies like to think they can kill me. I disagree. A friend watching your flank is all you need to stop these guys.
Notice how I left demoman out? I can't seem to beat him with any kind of efficency. He loves to lob the stickies towards me, and because of how slow I am, he can lob them from a long distance, and still hit me. There's no range in which the heavy has an advantage, unlike the soldier. The demoman can either through indirect stick bombs at me (and cause a great amount of damage), or use pipe bombs in close, direct combat (and cause even more damage!). I can't shotgun him to death, because his sticky lobbing can happen at greater ranges than my shotgun is effective at. So I have to generally leave the demomen to the scout, and the pyros. But there must be a sound strategy for the heavy vs the demoman.
As an aside note, I think that the heavy benefits the second most from crits. Demoman get the best with crit stickies, but crit minigun bullets increase there range dramatically. It increases the heavy's kill zone automatically, and unexpectantly.
EDIT: I'm going away from the internet for 2 weeks. I look forward to your responses when I get back.