Just two?

FFXIII-2's gameplay, FFVII's story/characters. Yes, please.

Alternatively, FFI's job system (ie, fixed jobs for the entire story, but selectable by user at start - replayability to extremes, that!) and FFVII's story/characters. That would actually be really neat. If there's one downside of FFVII it's that there are no real jobs outside of Aeris being a mage.

Finally, FFTA's job system with FFXI's MMO-quality. But FFXIV is getting to that stage slowly so I can't complain too much.