If you're looking to get back into the series, definitely buy a 2DS/3DS, as those are backwards compatible, and can play all of the Pokemon games of the last 4 generations (everything after Emerald). If you were a fan of Gold/Silver/Crystal, I would definitely keep an eye out for a copy of HeartGold or SoulSilver, as that's a great jumping point back into the series. It teaches you some of the mechanics changes from the older versions, and slowly introduces you to some of the newer Pokemon in the series. Omega Ruby and Alpha Sapphire are pretty much the same concept too, taking the old Ruby/Sapphire/Emerald, and adding some new stuff to it to keep the games fresh.
Outside of that, I would recommend looking into the newer games of each generation (Platinum over Diamond/Pearl, Ultra Sun or Ultra Moon over Sun/Moon), since they're more updated versions of the originals, and you won't miss out on much if you play the newer versions. The only exception to this is that Black 2 and White 2 are actually sequels to Black and White, so if you enjoy playing for the story in Pokemon games, I definitely recommend playing one of each!