The first ten seasons of the Simpsons are the almost unassailable peak of television comedy. Few things have come close, nothing has surpassed. Futurama is great and probably better on a dramatic level, i.e. deeper characters, more pathos, more emotional depth, but its simply not as funny. Similarly, South Park is better satire, but doesn't quite have the same hilarity as classic Simpsons, even if it is much more thought provoking.
The last ten years of the Simpsons, however, have served to almost annihilate its reputation. On balance, then, Futurama, but if you could cut the Simpsons off at the year 2000 then there would be no contest.