During my early days on TF2, I did 2 week runs for each class and the results state that I'm used to the playstyles of Medic, Spy and Engie. I did all runs so I won't feel so alienated with all classes.
Medic cause I just like the intensity of going to the frontlines and healing up my teammates. It's also a nice thought that you're the reason that this guy, who was supposed to die of burns, still lives. And because of playing lots of medic, I became more alert and more coordinated with the teamwork gameplay. Very fun
Engies cause there are times I like to lay back for a while and give support for my crew. There are times I do the turtling Engie when I feel laid back and times I do the Offensive Engineering which is one of the most fun things to do in the game. Build a lvl 1 sentry and you shotty everyone in the area to rack in the kills then move up!
As for Spy, well, I've always wanted to try this guy. During my first few tries, I always died a lot and it was pretty embarrasing to be the butt of many dominations (I have to admit, I got dominated by 6 people in one round O_O) but after match after match, I toiled and learned lots of Spy tips and tricks through tutorial vids, other Spies and experience in-general. Now these days, i can easily do my sabotage work and escape death (though I do get killed, it's just not so frequent anymore). I'm not bragging that I'm the best Spy ever... just that I played the class enough to be more of a reliability for the team.
It's also good to be a Spy/Medic/Engie mainer since as Medic/Engie, I can tell who are Spies just by seeing a player's behavior