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.
My Opinions Regarding the Pokémon:
1. Anime: OS > AG > BW > DP > XY
2. Movies: OS = AG > DP = BW
3. English Dub: 4Kids Entertainment/TAJ Productions (EP001-EP276; AG001-AG020) = 4Kids Entertainment (AG021-AG145) = TPCi/SDI Media (The Origin)/Very Good > TPCi/DuArt Film & Video (BW001-BW115)/Average > TPCi/DuArt Film & Video (BW116-BW142)/Poor > PUSA/TPCi/TAJ Productions (AG146-AG192)/Very Poor > TPCi/DuArt Film & Video (XY001-Present)/Terrible > PUSA/TPCi/TAJ Productions/DuArt Film & Video (DP001-DP191)/Atrocious
4. Games: XY > BW1 > BW2 > RSE > GSC > Y > HGSS > DP > RBG
5. Pokémon: The Origin: Excellent: 93/100.
6. TCG: Wizards of the Coast > PUSA > TPCi
7. Manga: RS > RB > Y > FRLG > XY > BW > E > DP > GSC