Sticking to the better series I've watched (avoiding spoilers):
[HEADING=3]THE GOOD[/HEADING]
THE WIRE: 5th season was a dropoff after the amazing 4th season, and I'm glad the show ended when it did, but you can't say there wasn't closure for all the major character arcs, plot lines, and themes.
SIX FEET UNDER: Almost a comical amount of closure. Also, if you've suffered loss, or are just a sensitive person, the last 10 minutes of the final episode are impossibly wrenching.
THE OFFICE (UK): Surprisingly sweet and sentimental ending to arguably the best comedy television series of all time.
EXTRAS: Nothing special to say, but a solid ending.
THE SOPRANOS: People can moan and whine all they like, I thought it was excellent.
[HEADING=3]THE BAD[/HEADING]
BATTLESTAR GALACTICA: It got lost in all the shark jumping the show was doing in its latter seasons, but the finale was appalling. Shlocky, meandering, pointless. Almost ruins the show, really.
DEADWOOD: It just...ends. It was clear the show wasn't close to being finished. Firefly fans complain, but y'all got Serenity. Deadwood just falls off a cliff.
BUFFY THE VAMPIRE SLAYER: Whedon did what he could with the final episode, but the damage had been done. The 6th and 7th seasons are disasters, and most of the 7th is borderline unwatchable.
THE SIMPSONS: END IT ALREADY KILL IT PUT IT OUT OF ITS MISERY FOR GODS SAKE