My favorite End Credits are the Red/Blue/Yellow and their FRLG counterparts. I like it because it seems to me like a celebration of your victory at the League and all you have accomplished in the game.
Well, I don't actually remember many of the credits of Pokémon games, though if I had to choose, I'd probably say the Unova games' would be my favourites. As for why, B/W's mainly because of that friggin' music and the fact that I have a bit of a soft sopt for those games and BW2's because it did the whole 'reminder of the journey' thing in a way that I rather liked, while also being backed up by some still-pretty-good music. Oh, and I do recall me liking S/M's credits, though I did feel they went on for a tad longer then they should've. Who knows, maybe I'll like US/UM's credits more when I finish 'em, only time will tell...
Also, I happen to have an irrational indifference to any credits that feature bysicle riding. I dunno why, maybe because I have an equally irrational dislike towards the bycycle themes or something, but still.
...Though if that poll included spin-offs, then I'd take PMD:EOS over those ones in an instant because nostalgiathe friggin' music again the main credits don't try to do anything too fancy like retellying the story or anything (probably because that stuff was done in the cutscene beforehand), rather they simply scroll over a sunset shot used previously in said story, letting you soak in and/or recover from the tear-jerking that PMD games tend to do towards the end. Oh, and (most of) the Special Episodes have their own credits music and background relating to the episode in question, giving each a unique tone or something, so that's cool. So basically, nostalgia.
The most memorable credits to me, were the Gold/Silver/Crystal (and therefore, HeartGold and SoulSilver) credits and the Black and White credits. For me, it's not the actual credits, it's the music that plays during them. Black and White had an amazing song for the credits, as did Gold/Silver/Crystal, making them memorable for me. Also, I really love the music box-like sound that plays once the credits are finished rolling in Gold/Silver/Crystal.
Gotta pick BW for the best ending theme, although, I like both DPPt and B2W2's ending cutscenes. BW's ending theme is the most fitting for its region, it's the definition of ending the main game with a bang. Also, the end of the theme has some sort of "setting up for a sequel" vibe to it, being menacing and all that.
LGPE and FRLG's credits are very similar thematically. I prefer the former for the cutscene screenshots and the ending with Pikachu/Eevee picking up flowers for Chase/Elaine's adoptive mom. Plus, the remix of the original music is much better than the FRLG version.
BW no doubt. It's not even a competition. I also like that the songs actual name is apparently "Onward to Our Own Futures" which is a nice title for the song after N, Hilda/bert, and Cheren and Bianca go their separate ways to live their own lives. It also just sounds really "epic rpg" which is very different for Pokemon. I listen to OOOF often.
I also like the GSC/HGSS themes a lot though, and listen to either based on if I'm looking for a more retro sound or not.