I play as Medic a lot of the time, mostly because I can feel useful as I increase my teammate's chances of not getting gibbed within the next five seconds. However, even though I like medic a lot, I've got twice as much time sunk into Engineer, because often my team is a bunch of offense-heavy thick-headed twats who don't realize that the best way to keep somebody from stealing our intelligence is NOT to try and steal theirs first. So me and the other two people on the team of sixteen who realize this all head down to the intel room or the bomb cart tracks or wherever we're going and build a bunch of beeping death traps. And then we fool a pyro with promises of cookies and milk or something to coming down and keeping the spies away. And by this time four hours have passed and I'm still playing Engineer. I'm okay with the class, I just end up spending a lot of time as it because I don't like to lose.
Also I pretty much don't play as any other class that sees front-line combat because I'm absolutely horrible in multiplayer FPS games. Any class I play as that's slightly slow gets their head riddled with Sniper bullets, and any class that's got less health than a dumpster gets gibbed by anyone with explosives. So I just stay behind the front lines and mop up the retard Pyro charges with two friends and a shitload of bullets, or hide behind my friends with the magic healing gun, praying my meat shield will hold together long enough to take down THEIR nest of sentries.
In case you couldn't tell, I really like TF2. But I only play vanilla(sort of, the Engineer's newfangled pick-up-your-shit-and-move-it doesn't count)! None of this item shit. When I get an item, I promptly give it away, and if nobody wants it, I destroy it. I really hate the item system and I refuse to participate in it. Well okay, this wasn't always the case, I stopped liking the item system when you could fucking BUY THEM. And the crates! Christ alive, don't get me started on the fucking crates.
Anyway. Tangential. Sorry. Those are my favorite classes, and I have way too much fucking time on my hands.