For me it’s rogue. Ive leveled every class to max multiple times over the years, and played them all seriously at one point or another.
Except rogues. I can’t stand to level one. The only max level I ever had was a free boost for BfA. I just can’t grasp the concept of their abilities, dislike stealth and always felt out of energy and just generally cant play it well at all.
mage, i use to love them, but every expansion seems to change them again, plus they are a revolving door of what spec is tops, its frustrating, and doesn’t have any of the characteristics that i enjoyed from back in MoP days, its not a bad class, in fact mages are usually wonderful choices, i just cant find the will or want to play them
Priest, mage, or rogue. I can’t pick between the 3.
Warrior. I’m not sure why, but the class has consistently never clicked with me the most out of them all, no matter how much I tried to get into it.
Shaman, just doesn’t vibe with me. Never was one for the elements, even IRL.
Warrior is second to last for me. Ive done it and sometimes they are fun but just being a dude with no special powers beyond getting mad and hitting things is just very boring to me.
I can’t play cloth DPS: Mage, Lock, Spriest
Mage, rogue, dk, dh, warlock, monk, Druid, pally, hunter, priest, and shaman.
Monk. I should love it. I like agile, fast, classes. I heal, dps and tank. I mostly play melee classes. But I just do not enjoy it. I really want to, but just can’t get into it at all.
Priest for me, idk why honestly. Leveled one to 10 I think at one point. But I just never cared for the class for some reason. But I also have never healed or cared for healing. Mainly tanked and dps.
Warrior, Shaman, Rogue, Druid, Demon Hunter, Monk, Priest, Mage and Warlock.
Edit: whoops, said Rogue twice. I guess I subconsciously really never want to main those 
Monk, they just don’t interest me at all
Paladin or Priest. Have max levels of each of them for multiple expansions and can never get into any of their specs/play styles.
Same. I used my boost for a rogue in WoD after only getting to about level 54 before that. I leveled him to max in legion and BFA, but never played him after hitting max besides professions. Then, decided it to use my BFA boost o. A new rogue, wondering if it was just the character or the class, but didn’t like new one either.
Had similar experience with Lock, but mainly due to class fantasy rather than class rotation.
I played all but those two on a regular basis.
But you’re good with the rest right?
Rouge’s, as they are a coward’s class.
I could never main a rouge. I was super invested into the class fantasy, the play style and being invisible, until i found out their resource is called combo points. What are they, 10 year olds? Surely the devs could have came up with something less cringe.
And also warlocks. One spec is gutted to the ground, while the other two specs have brain numbingly boring rotations. Detro spells never seem to generate more than 1/10000 of a soul shard at a time, and affliction’s dps cds don’t feel impactful enough to make it interesting.
Paladin. They felt so awful in Vanilla that I could never get into them since, even after many many revisions. It also doesn’t help that I kind of hate melee in this game to begin with and don’t like the “For the Light” iconography.