The South Park one was pretty good! I should go back and get that last achievement...
I LOVE some strategic TD. Like any genre, there's a lot of crap to wade through to get to the good stuff, but one thing I really like about this genre in particular is how broad the strategy can be.
There are some games which are there purely to give you a good time, but have extra challenges if you want to be more tactical (Kingdom Rush), some which are all about the calculations of upgrades and finding ways to game the system (Gemcraft) and others which are full of personality, story and great ideas (Defender's Quest).
I love all of those, but I also love games which are pure, unadulterated Tower Defense, and I think Defense Grid is the cream of the crop.
I am sure I own this one, but have not played it. The one game in this genre I have played was South Park: Let's Go Tower Defense Play! I like it, I should maybe give Defence Grid a try.