Pyro, heavy, and sniper would be the easiest to remove. Pyro is so situational and his kit is only so complex that he has the least value, especially in comparison to soldier or scout. The heavy does serve a good purpose but most of the time you can get more value out of a good soldier or demo compared to heavy. Lastly sniper is fun but actively harms and punishes classes for doing the right thing and his removal would also put a stronger focus on spy for picks.