Mabey I'm crazy, but I like the endings for most of the Mario games (the platformers). Sure, most of them just show a part of the map and a gameplay screenshot, but it's what happens before that that I like the most.. the parts where you know the Princess has been saved and makes a lame joke attempt, but at least you know she's safe, or the Yoshi eggs hatch. :)
The RPG endings have all been good. Mario RPG's was the best, with Paper Mario close behind it. M&L's was just kind of.. not as good, but still pretty good, and PM2's was not good. It wasn't bad, but I was expecting something way better.
Now, the spinoff titles.. I have never seen a Dr Mario game have an ending, but more of a congratulatory "thanks for playing this far" screen. The Mario Kart endings have been alright.. my favorite one has been the ending for MK64, and the worst one (in my opinion) is MKDS. All the character endings in the first Mario Tennis game were great, and I guess they were good in MPT too (I just hate the fact that Petey Pirahna is in that game.. his ending was just stupid). About the Mario Party endings.. they have all been good, except for MP's Advance, 6, and 7, with MP7 having the worst ending ever for an MP game (or mabey not.. I still can't decide whether MP 6 or 7 has the worst ending..) MP Advance only beats out those two for the worst ending because at least MPA's ending was fun! You could press a button to make the 4 characters dance!! w00t for interactivity!
Outside of Mario endings, I've also liked the Zelda and Metroid endings, as well as Chrono Trigger's (all of them), and, while it doesn't have an ending, Roller Coaster Tycoon is fun to play endlessly (but I guess that's sort of off topic.. sorry)