What do you think the hardest core pokemon games are? Which are the easiest?

Ultra Sun/Ultra moon are the hardest base games I think.
Easiest is probably Firered/Leafgreen.

Black/White 2 on Hard mode maybe? (It's been a while since I've played it so I could be wrong)
- Even if you used an action replay or something to use the key early, BW2 hard mode is actually a little easier.
  Because in BW2 you get boosted EXP for being underleveled. So fighting stronger opponents literally makes you level faster.

Colosseum is the hardest official one I've played. It's all double battles and some of the trainers have interesting teams.
- Skill Swap Slaking
  Nuff said
- I wasn’t counting this in core games, but for sure the hardest Pokemon game made.

I hear S/V is quite hardcore, a true test to gamer fidelity, patience and judgment by spending ~47 min in the first cutscene.

Hardest from an initial playthrough standpoint might honestly be diamond/pearl because they had pretty bad level scaling throughout a lot of the game. Once you've played through it once it's easy to prepare for the difficult parts though.
From a nuzlocke standpoint ultra sun/moon are also pretty hard because of the necrozma fight but again when you know it's coming it's honestly not that hard to prepare for.
I've never played through regular BW2 on hard mode but I've played through a sizeable portion of the drayano hacks (blaze black 2/volt white 2) in hard mode and those were pretty challenging.

Hardest I've played: BW2 on Hard difficulty (why they didn't make it a standard option in all subsequent installments is a mystery to me) Easiest: Alpha Sapphire and Omega Ruby. Literally almost impossible to lose, even when doing Nuzlockes.

I don't know if I found an exploit, but I think I beat the Elite 4 of Sword and Sheild with 1 Pokémon.
Ice Darmantian with either choice scarf or the other choice for attack.
Literally 1 shot everything. It was also drastically over-leveled

I've only played the Switch games in recent memory, and they're all pretty hard to lose. I did play older games at release, but stopped at White. I guess I could say that was the hardest because I hated it so much that it put me off the series altogether for a decade.
The only battles I lost in Scarlet were the electric gym, which I went to first when I assumed the game would scale based on badge count and I was using an all-bird team, and the first battle in Blueberry which I tried to win with only a Minccino & Hoppip (level 100 and fully EV trained and everything, but still weak as hell).
Sword and Shield were both also dead easy, and apart from the enormous spike at the end of Shining Pearl that was too - that was more tedious than difficult though. Having a 30-level difference between the highest wild encounters and the E4 remakes didn't make it harder, it just meant I had to waste a month getting ready. The rematches themselves were satisfying enough.
If you're asking because you want to get one for a child or something, I'd probably go with Sword.
- Only S/V battle I lost was the Eri boss battle in the fighting type Team Star base, which took several tries due to a similar assumption of level scaling. Didn't lose a single battle in Sword/Shield or Legends Arceus. Hardest Switch game for me was BDSP, just from needing two tries at the Elite Four due to the difficulty spike, and I barely beat them on the second try.
  Overall, in a general difficulty sense I think the most "challenging" core game I played was Ultra Sun. The totem pokemon provided a few surprises that made me have to resort to a few "plan B" options during battle, even if I didn't lose to any of them, and trainers sometimes actually showed a modicum of intelligence. I was also at around opponent level or even underleveled throughout the game.

My wife was having trouble with BDSP. She was getting walled by trick room in one of the early gyms.

Not sure what the hardest was for me but Scarlet/Violet have by far been the easiest for me. There was literally no challenge at all.