I can recognize that Seinfeld is good, but I didn't like the "average" episode all that much, either. At least at the time. But that's just me. The series finale is the world's biggest circlejerk, though.
If you ignore the last few seasons of Friends:
Chandler > Joey > Phoebe > Ross > Rachel > Monica
In fact, overall I still wouldn't change that ranking, but Joey, Phoebe, and Ross really got hackneyed by the end of it. So did Monica and Chandler too to a certain extent, but not enough to make me re-consider my opinion on them (but Monica started out last, anyways).
Rachel is kind of interesting because she's the only sitcom character I can think of that became less of a stereotype of herself and more of a realistic person as the show went on (or the opposite of Flanderization, to put it simply). Even what I consider "good" sitcoms don't pull that off, though they often have characters that retain the depth they started with. Of course, that alone doesn't make someone a good character, just as Flanderization doesn't necessarily make someone bad character. It's just odd that it happened that way.
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"One toot on this whistle will take you to a far away land."
-Toad, SMB3