I love Seinfeld (personal favorite is Kenny Rogers chicken), but I was thinking the other day that it already felt dated. I was in the sweet spot of the curve since it hit is prime when I was in high school and college) but man, its distracting to see the jeans w/white sneakers, the terrible cordless phones, etc. It's top five, but I personally prefer the simpsons. I think south Park is a sleeper, but its too bad because many people just think its all poop jokes, and don't know that it is very satirical.
The thing with Friends is, it could have been awesome. The first season or so was funny, and the Chandler/Joey stuff was usually pretty good. But it devolved into more of a soap opera. If it would have been able to straddle the line, it would have been perfect since both sexes would have loved it. But it became too girlie.