Futurama by a tremendous margin. It's more mature than the others, although that isn't saying much. It also has something of a continuity - a vast, tangled continuity - and that sort of thing really sucks people in. The gags are smarter and (in my opinion, at least) funnier. It also has moments that really connect with the audience. Don't get me wrong, The Simpsons has its sweet moments too, but I've seen nothing in any animated TV series that compares with the amount of emotion in episodes like Jurrasic Bark, Luck of the Fryrish and The Devil's Hands Are Idle Playthings, nor anything as clever as The Why of Fry, Roswell That Ends Well and Bender's Big Score.