I'm lookin for a high quality tower defense game

[JOKA_]

I'm doing a little research on tower defense games, and I was hoping some of you with a little more knowledge could help me out.

 

I'm looking for what maybe you would consider to be the top tier of tower defense games in 2 main areas; gameplay and graphics (If you think one game has the best of both that works too).

 

Thanks for pointing me in the right direction if you can.