Sun and moon is the only generations i have not yet played. Its mechanics put me off and it being 3D as well. Thread what is your opinion on the game.
I actually replayed base sun and moon recently. From a game play prospective it's great, a huge step forward from XY and ORAS in that regard. The region is pretty linear but there's still a couple side areas, though not a much as say the unova games. The addition of ride pokemon is also super convenient, and a genuinely great attrition (there's still a decent amount of areas you can return to when getting new ride pokemon, similar to HM moves). Storywise, I do like how team skull is just a group of thugs you'd find on the street, similar to team rocket. The true™ evil team the aether foundation is alright, this was the first time pokemon had a "twist villain" and lusimine in base SM is a genuinely evil villain even if it was because of influence from the ultra beasts. Despite flaws, I'd say SM is still pretty good, and GF was still trying out new things around this era. However the pacing is pretty bad with dialogue and cutscene spam.
I haven't replayed the Ultra games in a long time though, but from what I remember they are a bit effy compared to something like emerald and especially platinum. Gameplay wise, they are great, even better than base SM. They also extended the pokedex by 100 or so mons, which gives a lot more veriaty but doesn't make it overwelming. They also made move tutor moves more acceable due to easy bp grinding from the mantine surf minigame. They didn't really fix much regarding pacing though, still too much dialogue and cutscenes. The actually decent base SM story was made worse in the ultra games. Post game is better but I don't think is amazing as some people claim. Also USUM was made less than a year after base SM, so it was and still is often seen as a lazy cashgrab (I understand even though my opinion on the ultra games have improved slightly the last year or so).
I'd say play base sun and moon first, and after a while come back to the ultra games as to prevent alola fatigue,
Also fun fact, apparently East Asian pokemon players are way more negative on the ultra games than the western playerbase. Though East Asian player base is also more positive towards Scarlet and Violet than western players are, so "bad opinions" go both ways.