Republic Commando 2 or better yet Imperial Commando
This, just this. I honestly think Republic Commando was one of the most solid FPSs ever made, one of the most fun, and the only real thing wrong with it is that it was too damn short. Well ok, that, and maybe the AI was suspect at times.
I never fully understood why they never made a sequel to this game. It garnered universal acclaim from critics, Star Wars fans, and people in general. In fact, the only people I've ever met that did not like the game are people who do not like FPSs or Squad-based gameplay. It wasn't a huge title when it released, but that was because it was going up against some major competition at the time. But I do believe it sold well enough to garner enough attention and cash for a sequel.