The Sevii Islands. The Sevii Islands by far. It's the only remotely substantial content actually added to RBY, it actually adds new areas, new Pokemon, and new plot points to RBY.
I can't think of anything RBY does better than FRLG (LGPE yes, and that's actually having Wild Pokemon battles, but that's more LGPE shooting itself in the foot). RBY is such a basic ass experience that's outdated in just about every way that there's little if anything I can say it does better than any future game. And really, that's not a bad thing, since if a series is continually improving the later games should be blowing the older ones out of the water, but if you're looking to compare strengths and weaknesses between the original and its remakes, there are none to be had as far as I can see.