:3
Pyro is probably my favourite and 2nd most played Class. There's something satisfying about the nature of the Pyro, you have many ways to deal with many situations. Airblast is a severely useful tactic. Blast back rockets, grenades, arrows and Sentry Rockets (fun when you get it right). Uber coming? Juggle them in the air. Spies? Flame like you just don't care!
Enemies incoming, and you're certainly going to die? Set as many on fire and go down in a
Blaze of Glory. ehehehe.
Medic I also play often when mates are on, It's satisfying being a medic when you know who you are healing and they're not going to run off into the sun when you're under fire and trying to heal them. Most people seem to freak out when I get close. Most medics I know get scared and run off when they're hurt. I'll usually dodge around with my Ubersaw to finish up that Ubercharge.
I got my fantastic dodging skills from Pyro.
Third Played class is Engie, like the Medic, there's something satisfying about getting dispensers and teleporters up and supporting your team. The Sentries are just a fun bonus!

I love that the new Engineer update has allowed some flexibility. Engineers don't have to just bunker up and fend off spies and ubers. Particularly for attackers you can now help the push quite effectively.
I avoid the scout because I play on a 32man server, the spam is too much damage to handle.
I also avoid Demoman/Soldier if I can help it, I just don't find spamming rockets or stickies/grenades particularly fun, that being said if there's a sentry nest that needs to be destroyed and nobody else is suitable, I'll swap over to do it.
Sniper is fun, but these days theres usually already other snipers and too many means there isn't enough support on the front lines.
Spy is awesome fun, but I'm not very effective as spy so I end up avoiding playing him too much.
Heavy is awesome fun too, particularly with a medic. Quite satisfying when you're pushin the cart across 3+ capture points without dying and just taking everything out as it goes.