For Decepticons it's the original Soundwave. I love the voice, the mono lense eye, the cassette minions he spawns, and the fact he turns into a boombox instead of a military vehicle is baller.
For Autobots I'm going to cheat and pick Dinobot the Maximal from the old Beast Wars cartoon. I liked the redemption arc, his noble warrior personality, his weird spinning tail weapon, and the fact his raptor form is "generic dinosaur" in my head.