Edit. With months to reflect on it, I've upgraded my rating to 4 stars.
I have the same problem as PC Unfunny. Do I rate this show for it's halcyon years (say 1991-97), or what it's become today? I struggled to find analogies to guide me. Are The Rolling Stones one of the great rock bands of all time? Yes. Have they had a great album since Nixon was re-elected? Nope. Do they get 4 stars? Probably...
Ok, maybe this is a better analogy. You're reviewing a movie. The first half of it is brilliant, and the second half falls on its face. Does it get 4 stars? Definitely not.
Maybe just because PC jumped one way, I'm going to play Devil's Advocate and break the other, docking half a point for the later part of the shows seemingly endless career.
In the early to mid 90's, The Simpsons was revolutionary, revelatory and totally hilarious. They virtually introduced the techniquie of machine-gun gags and editing - every show had more funny moments than some shows have in a season, and you'd barely have time to catch your breath before the next one hit. Start trying to remember classic early Simpsons lines, and you soon realise you're going to need an excercise book to keep track of them.
Now all the original writers have gone. Groening bailed out with many of the best and did 'Futurama' (which is even better, and never jumped).
Most hugely popular, high quality shows face this problem. X-Files, Seinfeld, M*A*S*H*... but each of these shows only dipped in their last couple of seasons. Simpsons, by all accounts (I stopped watching it about 4 years ago), has a dip that encompasses half its run. So, with a deep bow at the shrine of those magnificent 90's seasons, I have to just chip the top from your score, Simpsons.
If the review were for sheer importance, Simpsons would get 4 stars. Hell, take 5, 10 stars. Anyone over 25 probably takes it for granted, but if your memory doesn't stretch back to the 80's, you may not realise how this show reinvented and rejuvinated television cartoons, and even the sitcom itself.
The 'animation renaissance' began with 'Who Framed Roger Rabbit' on the big screen. There were burblings in the late 80's, but the revolution really arrived on TV with The Simpsons in 89-90. In fact it first aired within weeks of 'Seinfeld' - the other show which re-wrote TV comedy rules for the 90's and beyond.
As far as TV cartoons go, a show is pre or post Simpsons. You can't get away from it.
Yes, I know this wasn't a review, but is there anybody reading this site who doesn't know what The Simpsons is about?