Best ones:

Blackadder Goes Forth
Cowboy Bebop
Six Feet Under (series finale)
Buffy the Vampire Slayer (The Gift - Season 5 finale)
Through the Looking Glass (Lost - Season 3 finale)

I don't have a worst one, but the final episode of Seinfeld has never sat well with me, it just feels so sour (even by the acerbic standards of the series in general). And I loved the show