Is this a real question? Everything but vMoL was basically cleared on the day it launched with hm the same time/a few days later.
vMoL took weeks to clear and HM still after 4 months hasn´t been cleared once. Anyone saying vMoL isn´t the hardest never tried it.
Even if the game doesn't directly call it such, MoL is basically a newer raid tier than the Craglorn trials and is tuned accordingly. Don't get me wrong though, I love the re-balance of the Craglorn trials, this is the first time AA and HRC have actually felt like proper raids and I'm very satisfied with the veteran difficulty of these trials.
If you were to exclude vMoL from the list I think the most difficult would be HRC, but I have not done SO since the patch so I can't say how the increased health on the Serpent has effected the difficulty of the encounter.
Zoning in a pve hub with your whole faction trying to dye costumes.
Seriously though, MoL is hands-down the hardest one.
Just so that everyone knows, my Altmer still can't have black hair. About a dozen of Altmer NPCs in the game have black hair. Just saying.
Meet my characters:
Command: Do the thing.
Zadarri, Khajiit Fist of Thalmor: The thing was done, as commanded. Durza gra-Maghul, Orc blacksmith: The thing was done perfectly, in the most efficient way. Tegwen, Bosmer troublemaker: You can't prove I didn't do the thing. Sings-Many-Songs, Argonian fisher: Sure, I'll do the thing... Eventually. Maybe. Aerindel, Altmer stormcaller: After extensive research, I've come to the conclusion that doing the thing would be a waste of resources. Liliel, Dunmer pyromancer: Aerindel said I shouldn't do the thing. Something about "resources". Gyda Snowcaller, Nord cryomancer: I will find a way to do it that won't waste resources and make Aerindel proud of me. Beatrice Leoriane, Breton vampire: I persuaded someone else into doing the thing. You are welcome, dear. Sahima, Redguard performer: Doing the thing sounds awfully unpleasant and really not my problem. Ellaria Valerius, Imperial priestess: I'll pray to the Eight for the thing to be done, if it is Their will.