My favorite game is The classic tower defence. It's simple, but very strategic. The creator really put some thought into the math and damage of each tower.
Yeah I like juggling in them :P. You make 2 long paths to the ending, and when they are almost to the end you open up 2nd path by selling a tower and block their current path, forcing them to go all the way back and through 2nd path. Repeat till all die :P.