Of what I've played, I enjoyed FOC and ROTF the most. I always see people saying FOC is worse than WFC, and that confuses me a lot. WFC was... longer, I guess?
For me, one of my favorite parts of WFC was the three-man team setup, which gave you a little more flexibility with who you got to play and how the level unfolded, but much more importantly gave the whole game a much more interesting, character-driven, cooperative feel, and one which FOC only really replicated on that great Cliffjumper/Jazz level.
Yeah, that's fair. What pushed FOC ahead for me was the fact that the fixed characters meant they could work the special abilities into the level design, so that you got a bit more gameplay variety in each stage.