Any of them are potentially good, except for Diamond/Pearl, which feature the slowest battles in the entire series and are pretty much superseded in every way by Platinum. Guess it depends on what you want.
Platinum is the best way to experience Sinnoh hands down, fixing D/P's issues with slow battles and a lack of Fire-types while adding the standard 'third version' content.
Heartgold/Soulsilver are amazing remakes of the Gen 2 games that incorporate aspects of Crystal too. They have beloved features such as Pokemon following you in the overworld, and share the Battle Frontier with Platinum if that's your thing. The only thing stopping them from being perfect imo is that they never really fixed Kanto's level scaling issues properly, but they're still amazing.
Black/White 1 brought us a more story-focused game that, at the time, broke with established concepts in the series. Of particular note is that there are no pre-Gen V Pokemon available until you've beaten the game. If you're looking for something fresh, BW1 might be what you're looking for.
Black/White 2 are actual sequels to the first game rather than just third versions, with a story that follows on from the first games. The story is arguably not as good, but if story is your thing, you might be better served playing the first games first. That said, BW2 have an incredible amount of content, including post-game content, and introduce older Pokemon back into the Unova dex with a pretty good selection all round.
My personal picks would be BW2 or HGSS (both of which are the peaks of the series for me), but you can't go too wrong with any of the games bar DP.
