I can't really say it's my "favorite," but I can't even guess how many HOURS of my life I've wasted on that damn Antbuster game (I know, it's old by now).
I also like Eternal Red: mix of tower defense with arcade shooter.
Yes, triple post and have a same thread on the same page. Again, it's unreal flash 2007 hands down, because it's the only game to me that has replay value.