>>38751805HeartGold and SoulSilver are the best ways to enjoy Johto if you're serious about catching up with the rest of the franchise.
Omega Ruby and Alpha Sapphire are straight upgrades to the originals, but are missing some content from Emerald. If you've got time, consider playing Emerald and then one of the remakes to appreciate the differences between the two.
Skip Sun and Moon for Ultra, and probably go with Ultra Sun at that because the forced time shift in the Moon versions is kind of annoying. Some of the story suffers, especially the bits to do with Lusamine, but Lillie is much more bearable in the updates and there's just more to do overall.
X or Y boils down to do you want to see a lion-man get killed (Y) or buried forever (X). The games themselves are, in my opinion, some of the weakest in the franchise, though, and you miss surprisingly little by skipping them altogether if you've already committed to playing Omega Ruby or Alpha Sapphire. I enjoyed them on launch, but the diminished player base and the fact that there's no more secrets left to uncover in the games made them difficult to replay for me.
Blacks or Whites: I personally prefer what White does with its exclusive areas, so those are the versions I'd recommend. In many ways Gen V represents a failed attempt to soft-reboot the franchise. The Unova campaigns make many breaks with series traditions in BW, but walk back quite a few of them in B2W2. Not that B2W2 are in any way half-assed apology letters, rather I'd call them the apex of the main series thus far.
Also: play Platinum. Diamond and Pearl are entirely outclassed by their update, which is another strong contender for best in the series.
In terms of order, I'd recommend starting with Emerald, then playing the DS games in order of release (Platinum -> HG or SS -> W -> W2), then Omega Ruby or Alpha Sapphire, and finally Ultra Sun. Take breaks, drop games if you're not feeling them, etc etc.
