I absolutely love Family Guy, but those of you who are willing to throw around "best animated show ever" over The Simpsons might want to consider how much more difficult it is to churn out 300 funny episodes than 50.
I could probably list fifteen comedies (sitcoms or animated) that were excellent for two or three seasons, but only one that has lasted nearly fifteen seasons - The Simpsons.
In fact, I will:
The Simpsons
Family Guy
South Park
The Honeymooners
All In The Family
Cheers
It's Your Move
Welcome Back Kotter
Taxi
Frasier
Friends
Family Ties
The Wonder Years
Just Shoot Me
Will And Grace
. . . and this leaves out several shows that most people thought were excellent that I happen to personally dislike (Whinefeld, The Cosby Show, MASH) and several others that were borderline excellent (Alice, The Jeffersons, Happy Days, The Odd Couple, Threes Company).
This is not to say that many (or even a few) of these shows were better than Family Guy; but rather, to illustrate how much easier it is to be excellent for 2 or 3 years than it is for fifteen.