I must be the easiest fan to please because I loved the endings of shows you mention like "Dexter", "DH", "Revenge" or, yes, even "Lost".
Of course, the worst series finale award goes to...
Even though I knew what would happen because it was aired in Spain one year later, and even with the well-chosen guest ex-stars, the "Conundrum" fiasco was even worse than all the other fiascos seen in the show since Ms. Reed showed off her bouffant in the 7th Year...Maybe it was inevitable given how much I had loved (and love) the series and its stars but...
Other awful endings (Spoilers if you haven´t watched these shows YET):
After killing the love story between Bruce and Cybill through her spontaneous abortion


in the season premiere, the show went downhill after a previous season with almost no Cybill at all and supporting characters with no interest. The last scene that you can see down here, though, was a reminder of what made the series the big success that it was for a couple of years or so. There would had been no "Ally McBeal", "Bones", or, indeed, "Remington Steele", without them.
After 5 seasons of talking and f....ng, that crazy Carrie girl from "Sex and the City" decides that she wants to marry the biggest jerk she has ever met. Great. In the video below you can see the 3 endings that were filmed, though only one was shown:
More to come...