The Tower Defense genre rarely gets the recognition it deserves because it's so overcrowded with cheap and badly make games. The gems are hard to find sometimes and it makes alot of people complain a new one comes out. It's great to see games like Gemcraft come out and revitalize the genre. Bloons Tower defense is another great one since it's so original.
Anyways, my favorites include Protector, Onslaught, Flash Element TD, and Vector TD.