Engineer, because I can be a all round engie, doing things that comforts my team without breaking my own fun times to be a solider to kill off others. Also, love how everyone's more liked class is pyro, its funny to me because Pyro and Engie are both classes that go togheter well.
Now my rant on why Free 2 Players need to learn all 9 CLASSES instead of being exclusives, lately, solider, spy, and pyro are being used by people who DO NOT know how to use each class even slightly to how their supposed to balance. I'll start with pyro and spy to begin, each are specialists at countering both solider, and the engineer, but pyro burns the spy like a roman candle, and it seems like no one spy checks the more obvious spies. This impacts the danger of spies taking points and killing off about to uber medics and heavies, along with defense Engies. Why Pyros need to focus more on Spy checking and ambushing is because I see WAY to many spies walk RIGHT ON PAST Pyros who would rather try to KAMIKAZE 5 soliders and 3 heavies instead of killing off the 3 spies he just witnessed kill his team. For Spies, learn to be sneaky and kill off key targets, act as someone crippling the enemy team. For Soliders, your not gods, learn how to bounce targets and how to dispatch each class, allowing for your team to take key points, try your best to kill off damage heavy classes, other soliders, demos and heavies are all targets that focus on damage, try to kill THEM and you will conquer the team by acting as a cripple in their plans. Go to wiki to learn more.