Devs recently resurfaced with a new game coming out this year according to their facebook, and I've been replaying some fo the games. I miss the MMO they made, street wars. Too bad that got shut down, oh well. Gotta say, sift heads 4 is my favorite. Instead of it being "wait til this guy shows up and shoot him" or just an action shooting mission, you had to strategize with your missions. Luring your target out, spotting people with very specific/hard body shapes and more. (Although, after your first playthrough its a pretty easy game since at that point you would know how to beat all the missions.)
What's your favorite one?