I voted some time ago but never explained. Here goes.Pokemon games are never really incredibly difficult. It's Pokemon. If you use the right tools, any mainline game can be easy.
But if I had to choose one to be the most difficult, it would be Ultra Sun and Ultra Moon.
All of the USUM Totems are insane. I've been unable to go through a single one of them without needing to come up with some strategy to beat them. They're all powerful Pokemon with great, sometimes ludicrous partners who synergize masterfully with the Totem. All the Totems also carry a stat boost; some carry an omniboost and one carries a double omniboost. They're absolutely a step above every group of Gym Leaders in the series and I'd argue they're above every Noble as well.
Teacher Emily from the tutorial has given me a harder time than the likes of Ultra Necrozma, Volo, BW Ghetsis, and BDSP Cynthia.
That Ilima battle from the early game is pretty intense for a fight so early. His Smeargle always has a super effective move for your starter.
Hau carries a stacked team of four Pokemon AND a Z-Move by the second island. He gets a team of five Pokemon including three fully evolved ones once you get halfway through the game, while still having his Z-Move.
Plenty of trainers casually carry good items including Z-Crystals. Some also carry seriously beefed up Pokemon for the part of the game you're in.
Both Guzma and Lusamine carry teams of quite powerful Pokemon. If I recall correctly they are EV and IV invested. Guzma's Masquerain can prove a big threat if you're unprepared for it. At least for Lusamine it's not a fight you're required to win in USUM.
You have to battle a legendary golden star dragon with dual base 167 offenses, higher base stat total than Arceus, a strong pool of moves with great coverage, and a whole omniboost in stats in case you thought it was too weak. It's very self-explanatory how insane this is and it has set a brand new precedent for difficulty in mainline games altogether.
I've had too many close matches with Giovanni from Ultra Moon specifically as his Mega Mewtwo Y has proven to be a major threat for my teams.
Lastly, Red's team is pretty beefed up here overall as well. I've surprisingly had close matches against him despite his team being lower level than in the Johto games.
ncG1vNJzZmiapaGvorPAq5uepl6jsrV706GpnpmUqHy1tMRmn5qqlJrAtXnPqKKepZ%2BjeqitzJ5ksqelq7JuvMuasJ6cXmeGdn2RbGY%3D