>>45823262I've replayed most gens about 5 or 6 times at this point, some as many at 8.
BW2 is the best, followed by USUM (if you can stomach the pace). The biggest reason is the Pokemon variety, but not needing HMs is the second biggest. You can have great teams with good type variety by the second gym in both. W2 having hard mode on replay makes it the absolute best, but that requires more than just the game + console.
Every other game gets hampered by lack of early Pokemon choices and needing to fit HMs in, but they're mostly all worth replaying. FR/LG really let you loose after Rock Tunnel (both variety and plot wise), so that one would probably be the best of the rest.
Worst is probably Sw/Sh or S/M, since both crawl at the start and really tempt you to put it back down before they get going.
If you are playing rom hacks, I really like HG/SS, best artstyle and following Pokemon are fun and comfy. There's also a ton of Johto mons that no one uses because they're shit or come too late, and if you fix that the team variety gets much more exciting.