Hmm... from when i first saw this show in early 02 (before this i wasn't allowed to watch much tv) to around 2007 i would've said yes. Since then it has bored me somewhat to see the same episodes over and over, and the newer ones are boring to imo. After all, stare at the Mona Lisa or Empire State Building for too long and you get bored with it, same with any masterpiece, however well made.
As to the actual question 'do i think it's the best American cartoon of the 90s' this is a tough one because i judge cartoons within each class or type, i.e anime, flash animation, etc etc and an overall judgement is difficult because you're taking in so many variables. However if i said yes i wouldn't really regret it... that is until i watch an episode of Stickin' Around (i imagine America refers to the continent not the USA) or Doug or Pepper ann or Recess or Rocko's Modern Life or what not. I guess my favorite show changes from time to time from the various ones on that high plateau of 90s cartoons, but when you say 'best' that is different to favorite and goes beyond my opinion to the realm of fact, in which case i would say 'yes' merely because The Simpsons was very effective, and was the best in influence, and thus the most powerful. It affected more people than any other i should think and has affected pop culture, how we look at animation, and how we look at our lives and society as a whole. It, being a satire based in realism (or once being such) was a major influence in culture, and who can't remember quoting a character once or twice during their lives?
So if you were to ask me whether it was my favorite i would say 'no' because atm Recess is, however if you were to look in an unbiased manner at all 90s American cartoons The Simpsons could be seen as the best by some criteria.