1. Metroid Prime 3: Corruption -- This is seriously one of the best games I've ever played (and I didn't even like Metroid Prime) and the controls just complement the fantastic game. And don't give me shit about MP3 being a FPA because I just don't care.
2. Counter-Strike source -- Counter-Strike was one of my first FPS games I really got into and I still love it. I know some people bitch about Source and it's not as good, but I really liked how the upgraded the maps and I just all around love this game. I can hold my own when playing with real people but I generally get slaughtered when I play online.
3. F.E.A.R. -- This was just a really fun game for me. Before I went into level design I was going to get a masters in AI so it was fun to see how this game worked (and even work with a guy that worked on it and even Deus Ex for you people that like that game-- I've haven't gotten around to playing it yet).
4. Team Fortress Classic -- Before I played CS I played TFC at the local gaming shop. I still have great memories of being a heavy and just camping in the opposite base pissing off the snipers.
5. UT2k4 -- I'm personally not good at this game but I like it just because it's so accessible and comes with a decent editor. The UT2k4 editor isn't by far my favotite (the UT3 editor isn't even my favorite) but it's still a nice editor that's great for beginners and experienced designers.