I too find Alyson Hannigan attractive, however her best role was Willow in Buffy. Lily is a close second though.
I cannot stand Friends, I really can't. This gets me plenty of flack among my mates, but honestly I can't stand those characters. The only reason they're friends is because no other group would have them. Personally haven't seen Seinfeld, it dropped under my radar somehow. For me, the best american sitcom is Fraiser.
We brits do have the very best sitcoms, however, it's the one thing we do impossibly well. Blackadder, Spaced, Outnumbered, Black Books, Vicar of Dibley, Father Ted, IT Crowd, The Office - the list goes on.