Shadow Complex is a classic at this point—I played through it at release while sick in bed with H1N1 in 2009. it's like the perfect medium-sized game—it goes above and beyond to do cool stuff, without ever overstaying its welcome in any regard. just enough neat cool things you haven't seen before—like pushing the right stick up contextually aiming your gun into the Z-plane to shoot guys in the background. I wish people set out to make more medium-sized games like that, though I'm sure the budget was much higher than we think it is.
I hadn't realized it came out so long ago! It's gameplay is evergreen.
Thinking of medium sized games, I think the original Orcs Must Die! would qualify as well. It was a tight, solid experience. There aren't too many 3d tower defense type games that I'd put on a must-play list, but OMD is at the top.
