Although I didn't know at the time it was to be his last match, Steve Austin's final match against The Rock at WM19 was the perfect way to end his in-ring career IMO.
Agree with Flair, that was a very well done storyline.
Kevin Nash's final WWE departure was fairly lacklustre. He was the first guy eliminated from the Elimination Chamber at Summerslam '03, then I think just disappeared after that.
Brock Lesnar and Goldberg - 'nuff said.

And I guess depending on your point of view, Bret Hart's WWE departure was one of the best or worst ways to end a career....