I'm surprised no one has mentioned Desktop Tower Defense In my mind it is the best TD game, and to whoever said that Bloons Tower Defense was the first TD game... uh yea not even close.
I also love Tower Defence games! espiacally Bloons TD 2...... but my friends says(in real life):"all you do is put on some towers and you dont do anything!" but i still like tower defence games.... :P
Some people think they are boring but If you think that, I would encourage you to try some more. You WILL find one that is good. The genre is so large and sometimes junked up with copies that it is sometime hard to find the gems (No pun intended) of the genre.