I would chose Pokemon Emerald personally, it's gameplay far passes ruby and sapphire, and it would be fun to play that before Omega Ruby and Alpha Sapphire. Team Magma and Aqua are some of the best criminal organizations to date in a pokemon game, the fact that you have to pick a side makes it so fun. I enjoy the pokemon from this generation a lot, and although some don't like it that the region is half water, i think the surfing music makes up for that (and some pretty awesome new water types).
Firered and Leafgreen are great too, they are so nostalgic yet have a new feel to them (2004 "new"). the post game is nothing to laugh at either, i felt like the post game in Ruby and sapphire was lacking, though emerald brought it up a few notches giving us more fun after we thought it was all over. I feel like the art design is much more sleek that RSE.
If you played RBY nostalgia will play a role for FRLG, if you want a new adventure RSE is your guy/gal. Again i would choose emerald and that is due to the fact that i grew up with Ruby and Sapphire, and Emerald is better in my opinion.