[Mr Cat Dog]

Discounting people like Jon Lovitz/Albert Brooks/Phil Hartman who did multiple episodes (and multiple characters), my favourite would have to be Meryl Streep playing Jessica Lovejoy in the episode 'Bart's Girlfriend'. I didn't realise it was her playing the role until a few years ago, and she sounds really convincing as a 10-year-old girl without having a background in voice acting. While most of the guest stars before the show turned into a big steaming pile of turd did 'act' when voicing their characters, Streep's turn felt like a real performance. So much better than that Sarah, Plain and Tall!