In terms of gameplay, you can basically go for any of them, since they're the same game with a new but ultimately minor gimmick each time. In terms of graphics, go for one of the newer ones, like Diamond or Pearl. Unless you like pixels, in which case go retro with Red/Blue! If it's grind you're worried about, I still think you could pick any of them. I have never felt compelled to 'catch 'em all', whether there were 150 or, what is it, 400-something now? You just catch and train what you like and start a new game when you get bored.
Sorry to be confusing - basically I would go with Diamond/Pearl becuase it's the same game with better graphics and more choice (sort of) when it comes to the Pokemon you can catch and train. =)