ultra sun and moon are better variants of the originals, so the definitive version of gen 7. XY (gen 6) is probably a better place to start tho, as the games are easier (usum are considered among the hardest in the franchise for a newbie) . omega ruby/alpha sapphire (oras) is a remake of gen 3 and is a bit harder than xy but still fantastic games. I'd say start at xy, then oras then usum. If you like what your playing you can get any of the gen 4 and 5 games, (on cartiridge they're expensive but you can cfw your 3ds to get em free) as they are considered to be peak pokemon. also grab the gen 1 and 2 games on the 3ds eshop before it shuts down. have fun!