If you've already played the major 1st and 2nd gen games, and aren't too excited about the DS games (which is too bad, since they're all pretty good), I would probably suggest trying Emerald. It nicely combines the storylines from Ruby and Sapphire, plus you can participate at the Battle Frontier- which I think was one of the greatest additions to the main franchise games as a whole.
If you do have a DS, I think it would be a wise decision to some of the games there are to offer. If you enjoyed Crystal, HG/SS are very good remakes, with lots of extras and upgrades along the way. Platinum and the 5th gen games also show great improvements over their predecessors, and are strongly worth a playthrough.