Gen 5 would probably be the most interesting, just because if you go to gen 6 or 7 you have access to ORAS, which gets you all the hoenn pokemon without needing any transfers. Also, you can do up to gen 5 without using any software other than the games themselves; transferring from 5 to 6 requires a transfer program external to the pokemon games, and 6 to 7 requires that you use that online box thing.