...First off, Emerald is superior to Ruby and Sapphire, due to all the additions (Battle Frontier, both teams and their matching Legendaries, Kanto Pokemon, etc). Second, as far as which game to get, it depends on Pokemon preference. They each (naturally) have Pokemon exclusive to that version. Whichever game that has the more favorited exclusives is the one to buy.
As for what those exclusives are, I'd suggest checking out the FR/LG FAQ in the matching subforum, as...I can't remember right now :P.
And yes, either FireRed or LeafGreen along with those other games you already mentioned is good enough.