Nostalgia is pretty much the only reason why I voted Gary as my favorite rival of Ash's (this time, I am being nostalgic about something and don't really prefer it for valid reasons), but truthfully I feel Paul is his best rival.
I still love Gary and find Ash and his rivalry very sweet and cute, but Paul and Ash's was so much more dramatic and heartfelt, so that one is the best one and one of the best things about DP in my eyes.
I liked Trip and Ash as well, but the writers ruined Trip's whole character in the Unova League, which was sad to watch.
"If Pikachu becomes a Raichu, I might be able to win that badge. On the other hand, to make it evolve just to fight...I would be just like Surge...Pikachu, what do you want to do? I really wanna beat that Surge, but I also don't want to force you to evolve if you'd be happier staying the way you are now." -- OS Ash, ESS
1. The Anime: OS > AG > BW > DP > XY
2. The English Dub: 4Kids + TAJ Productions (Seasons 1-5) > 4Kids Entertainment (Seasons 6-8) > DuArt Film and Video (Seasons 11-Current) = TAJ Productions (Seasons 9-10)
3. The Games: XY > BW > RSE > GSC > Y > HGSS > DP > RBG
4. The Manga: RSE > RBY > BW > DPPT > GSC
5. Pokemon: The Origin: Excellent: 94.7/100.
6. The Movies: OS > AG > XY (Prediction) > DP = BW