Last episodes are tough to get right. They have to match the tone and the style of the show and yet go beyond it. They have to resolve enough issues to keep you satisfied and yet not try and tie up every loose end and become drawn out and convoluted.
Most of all, I find that a great last episode of a TV show should stick with you. It should really resonate and keep running through your mind for days after. It should leave you sad that you will never see those characters again in new situations and yet happy that it was so excellent.
Last night I finished Battlestar Galactica and though I see some people weren't too satisfied with the ending, for me it was perfect. It got the tone just right and I can't get it out of my head. It's up there with the last episodes of Spaced, The Shield, The Wire and Arrested Development. Every time I go back to these shows, their last episodes deeply affect me. Cracking TV.