Best/worst TV season enders/twists

Great twists:
Buffy's mom dying.

The episode on Scrubs where they make a big deal about 1 in 3 patients they see dying, and introduce you to three patients. Then at the end, all three die.

Same Scrubs trick done even better: The donor had rabies and they all die, prompting Cox's meltdown in the middle of the ICU.

"I made you eat your parents! I made you eat your parents!"

Terrible twists:
Roseanne winning the lottery
Roseanne not really winning the lottery

Terrible ending:
Lost, of course.

Great ending:
"No you don't, but thanks for saying it." Spike

 

The Omar Epps twist in ER was pretty cool.

Epps played a young intern who didn't show up for work one day. Then, a young man who had been hit by an El train was brought into the ER, his injuries rendering him unrecognizable.

They tried to page Epps' character, and then the patient's pager started going off ...

 

Which was then ripped off years later by Grey's Anatomy.

 

Speaking of Law & Order, the episode where all the characters witnessed an execution then ended with Lenny falling off the wagon and Claire Kincaid's car getting t-boned as she was taking him home ranks up there with the best of them.

 

"Mr. Worf....... FIRE!"
That will always sink with me as the best cliffhanger in TV history. I was really young and it was the first true cliffhanger I ever experienced and was sucked in by.

Lost did it over and over to me, and I didn't completely hate the ending like most do.
Breaking Bad is doing it really good again and again.
South Park did an amazing job with "Cartman's Mom is a Dirty Slut" and an even better job when instead of the payoff they aired the Terrance and Phillip episode. They played with that again at the beginning of the second episode of Cartoon Wars, and I laughed for the rest of the episode because of it.
South Park also has that joy of having two season finales because of the two runs of 7 episodes each, so they get to do things like they did this last run and leave a very wild cliffhanger and even though you know 7 more are coming this fall, you have to wonder what's going to happen.
"Who Shot Mr. Burns" was good and bad at the same time, with a letdown ending to the cliffhanger payoff.

 

The Producers season of Curb had the best ending of any season of TV.

Seinfeld should have ended after Susan died. All of them moving past the death in about a minute and going off for coffee would have been the perfect ending.