I vote for Emerald, even though Gold was my first game and I played Blue and Yellow before third gen, because:
-The plot is way better in RSE. In Firered and Leafgreen, all you're doing is taking down Team Rocket, and that sort of comes to a lackluster climax anyways.
-Sceptile is my main man. It sometimes pains me to start any RSE run with a starter other than Treecko.
-Battle Frontier made for a great after-game challenge. I preferred Emerald battle frontier to those of DPP and HGSS, probably because it was bigger and more spread out, and had more stuff to get into (such as the cave full of Smeargle, which I was absolutely convinced would have Mew in it after that little girl said she saw something with a long tail hiding... T_T)
However, Firered does have the Sevii Islands going for it. I really enjoyed those, it was a lot of fun. I raised my female Charizard to level 100 there :') and then deleted her by accident. T_T