I like all the classes. It’s hard to pick a favorite. However, the three classes I have played the least are the spy, demo, and pyro. I usually pick whichever classes the team doesn’t have or needs more of. I try to think about the team. 
But yeah, usually we don’t need so many spies, though, and since the pyro update came out, I think 1 or 2 of the servers I play in have restricted how many pyros you can have. I need to work on the demo some more. I think I have less than an hour of gametime with him.
Also, I can see how people would think the cartoony graphics are supposed to appeal to younger people, but the game’s rated M (which is 17+). Assuming people follow the ratings, do you really think the graphics are what hook 17-year olds (and up)? I’m way older than that, but for me, it’s the gameplay. I do like the cartoony graphics, though, haha.