They are all great in their own rights.
D/P: Good Story, Easy to get a hang of, New gameplay elements that make the series so much better (Physical/Special Split), overall a good time.
Pt: Even better than D/P. Pretty much everything from there remains, but with a lot more (areas, secrets, etc.). If you're gonna get D/P, just get Pt.
HGSS: Everything that made the Gen II games good, made into an awesome remastered adventure. The only new thing I noticed was the Pokèathlon, but Gen II was great to begin with, so it's also really good.
B/W: A bit of a deeper and longer story, Introduces many new things (Triple/Rotation Battles, Lots of Pokemon, etc), and overall a fun time. The only problem is that you can only catch Unova Pokemon until you beat the game.
B2/W2: Well that problem got thrown out quickly. Anyways, It's just as deep and interesting as B/W, the Pokestar Studios and PWT are all very fun, White and Black Kyurem are very helpful (and hard to beat I should add). I guess 2 years later, things really DO get better!