(This is most likely the first and last time I do such thing, so don't worry. )My most often played class is the Medic, but sometimes I have "Scout days", "Pyro days" and things like that, making me think about which class is the most fun to play. Recently, I kinda rediscovered the Demoman, coz I found a lot of enjoyment in it.
Of course only I can tell which one I like the most, but I wanna ask for your tips too.Actually, I already asked "the average person", thanks to the Valve stats:
My research: the most fun class in TF2 by the average person
I thought that having the power of Maths could easily decide the question for me, but wasn't really that helpful...
In short: I think it only confirms that the Medic and the Demoman will be my new favourites, keeping the the Engineer and the Soldier also on my list of frequently played classes...
Oh and Spy is not really fun on a server with 32 players, you must be pro to really enjoy it. Also, I still have the backstabbing problem.
It's gotta be the Medic! The satisfaction you get when you just done a Über that made us win the game, and the fun your having when you leave people to die is truly hilarious!
Soldier is awesome too because of the high Damage done by the rockets, and his 200 health.
Scout too cause of his totally quick movement, jumps and FaN ;P (and taunt kill)
Demo for camping and surprises too ;P
Heavy for 300+ health and taunt kill
Spy for even more camping and instant kills! 
Sniper is Hard :/
Pyro is just for rocket reflections 
