Lots of great answers, and there are good arguments for all of them. For me, I'm probably looking at it from the perspective of "what do those in the know think", and many of them will say nobody was in the same class as Chip for cash games, and Stu Ungar for tournaments, though he's also widely regarded as the best gin rummy player ever as well.
That said, there are a lot of great players today as well, especially with how much
online poker has grown.
I agree with an earlier poster in that it would have been nice to see how Stuey would have fared against modern competition (he'd be 61 today).