Okay, so I've been on a bit of a kick with these lately. I played Plants vs. Zombies and Crystal Defenders. I have the gold box of Zombie Pirates sitting here beside me for when I get a free moment. What do you guys think are the best?

Note: I fully intend to pick up all of the Gratuitous Space Battles stuff at some point.

Hey Orc - If you really love these Tower Defense Games you need to check out this site. Good Luck.

Oh great, I should just insert a caffeine IV now.

I really like Defense Grid: The Awakening. Check out that one:

http://defensegrid.hiddenpath.com/

There's a demo on Steam:

http://store.steampowered.com/app/18500/

One of the best tower defense game I have ever played. Highly reccommended! :)

http://www.astraltowerdefense.com

based on, and created by the people that brought you

http://www.pardus.at

Try Immortal Defense for a fresh look on a genre.

Also, flash games "Protector" and "Cursed Treasure: Don't touch my games" are really good.

Sol Survivor is some really good fun. The multi-player is good too but you need to usually organize a match to get one started. It has more possibilities for tower combinations than Defense Grid(which I also love btw) but it doesn't have mazing.
http://www.cadenzainteractive.com/solsurvivor/
It's on Steam and Impulse for ten dollars

Another vote for Sol Survivor and Defense Grid. There also Dungeon Defenders coming out soon its kinda like an aRPG with tower defense so you might want to check it out.

Dungeon Defenders is a new tower defense/ARPG hybrid. Demo will be available on their site in about a week.

My Favorite is Plants vs. Zombies.

I like the kind of humor that appears in the game.

I am not a huge fan of tower defense games, but I must put in a vote for Defense Grid. A friend gifted it to me a good while ago, and I was pleasantly surprised with it. I found myself replaying levels even, trying for the highest score possible with no survivors.

Definitely worth a look, and I believe someone mentioned above, there is a demo for it as well.

Here's another vote for Defense Grid. I never cared for the genre but had 800 MS points left and my brother accidently bought it instead of downloading the demo. I figured that since I had it on my 360 I might as well play it.

It quickly became one of my favorite Arcade games and I was addicted to it for a good month before I got hooked on Borderlands. It's just a great game, easy to play but hard as hell to master, not to mention that if you include all the challenges and gameplay modes for each level it's a huge game.

I actually picked up Defense Grid on the Live sale this week. I've been cracked out on it for a couple days. This game is really fun.

And you know it, I got Gold on level 18!:)