Plants vs Zombies is pretty great.

Defense Grid is really pretty great in my opinion. This is my recommendation.

One of my TD-loving friends likes Prime World: Defenders.

Anomaly: Warzone Earth is one I'm sure you own from Bundles, it's a reverse tower-defense which is briefly kind of entertaining, but not actually executed all that well.

Dungeon Defenders is only marginally a TD and is poorly balanced, but it's still fun if you want multiplayer.