I'm not clear on whether UM/US are sequels like BW2, or third versions with minimal changes, so that's something to think about. You may want to wait for more information about those before making your decision, if you haven't already gone and bought a game since making this thread.
Anyway, from what you've said, you're a big fan of gen III, and I think that's worth considering. If you played those games when you were younger, nostalgia is powerful, and I would actually recommend picking up one of the ORAS games first. From your brief initial post, I wonder if you feel like you should get the 7th gen games because they're newer, but really want to experience Hoenn again. And even if there are some superior elements to the newer games (personally, I've enjoyed gen 6 more than gen 7, but that's probably mainly because I haven't gotten to put the time into SuMo and because I tend to like the regular gym formula, which is missing from SuMo), you may find that nostalgia is enough of a factor to tip you into liking ORAS a lot.
So, unlike a lot of other people in this thread, I would say to start with ORAS and get your fill of nostalgia. From there, if you're not huge on gen I references (unlike me), you may want to skip X and Y for now and move on to either Sun or Moon. They're definitely worth playing, but they're different, and balancing a "classic" style Pokemon game with a more unusual one may be fun. From there, if you have the urge after that, go back and fill in the gaps in the series.
The one caveat is if you want to participate in competitive play right away. If you want to start entering tournaments, you'll need a newer game for that (Sun or Moon).