Any will do really.
Crystal is probably better to start on if you're going to start on an old game.
But you could always try FireRed/LeafGreen, or the 3DS re-release of Red/Blue.
Or if you want to start with Gen 2, then HeartGold/SoulSilver aren't bad places to start either.
I personally would choose Emerald as a place to start, but actually, I may even favour just going with OR/AS.