Just get them all!
But what was the last one you played though?
Personally, I thoroughly enjoyed Diamond/Pearl/Platinum.
Platinum is probably the best bet, since it's got a longer and slightly more interesting story line, as well as all the version-exclusive pokemon.
Black/White were good as well, in my opinion. I quite liked the storyline, but it didn't quite seem (for me) that it took the focus of my journey.
Black2/White2 were really really good as well, but the plot there seemed more interesting, but not as memorable or exciting, really. It just seemed like I was being carried alongside it, rather than playing it.
But I suppose I took the game slowly. Set myself personal goals before each gym and whatnot, so I took longer to get through it all.
HeartGold/SoulSilver, also very good. It's fun to walk around with your pokemon with you.
I felt it was a lot of fun for me, because my first game was Pokemon Crystal, when I was younger, and I really couldn't remember lots of the storyline or gyms, since I was too young and it was a hand-me-down.
If you played Gold/Silver/Crystal then it'll probably be a good nostalgia trip too.
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