Heartgold and Soulsilver give you the most bang for your buck. As as far as I know they're the only games that have two area's. Both the area's from gold/silver and red/blue.
Alternatively, x and y are good. The game is less challenging than earlier ones, but has a lot of things that previous games didn't have. Plus, it's also a HUGE step up graphics-wise.
Alternatively, x and y are good. The game is less challenging than earlier ones, but has a lot of things that previous games didn't have. Plus, it's also a HUGE step up graphics-wise.