Without a doubt Maul. He had so much fantastic character development in the animated shows. Maul was so successful in his plans that he had an entire planet under his control and Sheev himself needed to shut that shit down. And then Maul was still up to stuff all the way past RoTS.
Then they gave him one of the best endings for a Star Wars villain as well as one of the best screams:
(A scream so good they gave it to him as an emote in Battlefront 2)
And sorry, but Jango is cooler than Boba. Better color scheme, better design, dual pistols, got a cool fight with Kenobi, and actually used Slave I in a dogfight.