The answer is the simpsons. Other shows - especially futurama, personally - have hit the same highs at their best, but the simpson have more genuinely great episodes than most shows have total episodes.
One that i don't think has been mentioned, and probably isn't as widely seen since it is a "kid" show, is Gravity Falls. It is a top 5 all-timer and i'll die on that hill.
I'm assuming looney tunes, animaniacs, and the like (shout out to bluey, too) are not in the conversation. It is a weird distinction, but to me those aren't what i consider a "series" in the context of this conversation.