SPOILER AHEAD FOR SIX FEET UNDER
If you have legitimate plans to watch the series soon, don't read on. Otherwise, I don't spoil everything. Just the end.
How will the series end? Will it suck? Will they tie up loose ends? Will the characters be redeemed? Some shows miss the mark (ahem, Battlestar Galactica and Lost). Some shows get it just right (Cheers and arguably The Sopranos).
Occasionally, it's perfect. The absolute best series ending of all time was for Six Feet Under. Not only was it beautiful, not only was it emotional, not only did it complete the stories of the characters, but it did it in a way that was SO PERFECT for the series before. Spoiler: A show about death showed us the deaths of the main characters. And the song playing, Breathe Me, makes it perfect.