Steins;Gate is a fun time-travel series, but you have to watch the entire thing to truly appreciate it ^^ the characters are what makes it fun to me. You got the main character as a self-proclaimed mad-scientist after all
I also like Monster, which focuses on a brilliant Japanese surgeon who does the morally correct thing only to have it backfire on him. The monster of the series, the antagonist, is the strong point here for me.
Also Death Note and Code Geass are popular ones too unless you simply forgot to mention watching them!
Ah forgot Full Metal Alchemist: Brotherhood