In terms of learning and importance. Yes, I'm sure the "pros" will heavily argue against my listing but that's in terms of "ease to learn" and "useful on all maps/modes" importance combined.
Idk what you mean by that, or if it's a potshot or what, so I'm going to ignore it.
Most classes tend to have multi-uses available, either in attack or defend really. What I reccomend doing is alternating between valve public servers and other servers like Lion's Den/Gibbed or others like that as it is more challenging so you can not only learn the mechanics but play at a higher level in general.
Also adding people from here is a good idea, most play tf2 regularly and are more than willing to help you or offer tips and other ideas.
The Frontpage of this thread provides a simple breakdown of each class, their strengths and drawbacks.
Another tip I'd give you is, unlike most FPS your Kill to Death ratio isn't really as important as you think it would be. This is a team oriented game and although it's forgotten by many players it ultimately doesn't matter how many kills you get cause if you lose the objective...well your team loses.
Personally my favorite class is the Pyro, I think I learned scout and demo first, never really got the hang of Soldier and Spy. But like I said it's different for each person, and you get the hang of different things. I personally find medic and engy a really boring class but some people excel at it, and are fun to play with when they are that class and it is super appreciated.
The best way to find out, is just to play the game! Add people on this thread, hit them up and just have fun. You'll probably die alot, but that's just kinda part of it initially, actually, even when you get better you die alot. But it doesn't really matter cause you respawn.
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