Black 2. Its challenging and filled with cool moments, has the best music and hands down the coolest title legendary. Also Platinum is great but it will kick your ass, really!
Actually, Red, Blue, Yellow, Gold, Silver and Crystal are avalaible on 3DS Virtual Console.Diamond/Pearl or X/Y would probably be good starters since getting a hold of the gameboy ones would be pretty hard unless you have an emulator of some sort.