For me it would have to be LeafGreen
But I guess since i've been playing pokemon since I was like 6, I like the fact that they brought back the classic game. Emerald is really good and I remember anticipating it and being like "OMG BATTLE FRONTIER" but to me the Sevii islands and all the original pokemon mean a lot more. I'd say if you started pokemon at a young age and where brought up on Red/Blue get Leafgreen. If you don't really care, then go for Emerald, however I feel like you miss the importance of pokemon unless you start out with red/blue or firered/leafgreen