My favorites were Prey of the Clan, 10k ways to die, perhaps Rescue, and Extinction Event.
I like 10k ways to die because of the wide array of weaponry you get to use, challenges you face, enemies and their weapons, and the simple number of enemies you get to kill! Have to admit though, first time I played it I hated it!
I haaaaaaaaaaaaate Under the Gun so freaking much! I played it so many times just to pass it, then I had to go back for an achievement!
Bug Hunt and the one after that were somewhat fun, but all the friggen tunnels, pipes and all that was nerve-wrecking! Not to mention you had bugs right on your tail 95% of the time. It was pretty fun though to place mines while your climbing a wall with them behind you, and they drop like friggen flies!