hmmmm....
1. Soldier - used to play lots of Ace Combat, so i love the feeling of predicting my enemy's movements and shooting that rocket/missile, blasting them to kingdom come (and also going sonic boom with a rocket jump)
2. Heavy - Once again with Ace Combat, i used the machine gun on my plane plenty of times when a good L0ck-0n can not do the job. Just get on 2fort or Black mesa when i do, and you will see my minigun blazin! (also enjoy my cheeseburger skin)
3. Demoman - Well this is the class that does not require my Ace Combat skills...but i do enjoy setting traps on edges, giving a split second "GASP" moments as people get an explosion in their face. Also the power of gaining ground by laying some stickies is just as rewarding.