For me it’s this beauty, no contest: https://fr.wowhead.com/item=95041/serpent-nuage-du-gladiateur-malveillant#screenshots:id=357479
I also regret not farming the Legion S3 elite set for warrior: https://www.wowhead.com/transmog-set=2374/cruel-gladiators-plate-armor-elite-recolor
Now this looked savage.
What about you?
The MoP challenge mode mounts.
Could have gotten one, was just too lazy to put in the effort, had all the time in the world to get it done.
Then they made it so if you had one you’d get the other 3 for free too. 
Still for some reason didn’t motivate myself to get it done.
Meh, they’re fairly bland. I don’t regret missing out on them.
I do regret missing out on the BFA pre-patch mount for the Alliance though. The hippogryph looked great.
I had a real shot at getting the s4 gladi drake, but I kinda meh’d out because I cared more about camping people in IQD.
I also regret not buying the wrathful tabard “because it looks bad” even though I could’ve.
I wish i wasn’t too lazy to go for field medic
That was a proper grind. And I did it literally the day before pre-patch 
For me, it has to be the Grove Defiler.
Wasn’t around during WoD, and it’s the only mount that elicits a pang of envy from me any time I see it.
The Gul’Dan transmog staff.
The epic nightsaber mounts in Vanilla WoW, early after release, before they got updated with armor on.
I had the gold for them, but chose to save it for the Winterspring Frostsaber mount, which I was more interested in.
Those unamored nightsabers though, especially the white one, are rather unique today. Hindsight and all that… 
Lol, imagine saying mop cm mounts look bland and then praising the most basic and boring noob mount in the game.
I would delete the hippo from my acc if I could
There is only scarab lord
all the cheesed titles from wotlk/cata with server transfer,scrub lord, black qiraji cockroach, realm first etc. zg mounts, zg tmogs
I’d rather they bring back removed content. Like MoP/WoD legendary quest chains and Legion Mage Tower. Don’t care about the rewards.
No regrets here but glorious tyranny enchant would be nice.
% Death’s Demise and and % the Immortal
Such cool titles should never have been removed 
Cycle Armor - Druid MoP CM set
The only good druid set ever made
Definitely a fan of the paladin challenge mode armor from Mists of Pandaria, but I didn’t play my paladin at the time, still enjoying my other transmogs though!
I don’t know what I’ve missed and therefore I have no regrets 
Benediction/Anathema.
The Anathema staff model in particular is such an iconic representation of the Shadow Priest.
Atiesh, i know khadgar has it, but i really wish i could get it, even for a artifact reskin.
I miss the Key Chain, then of course Mage Tower and Challenge Mode transmogs/mounts.