You know, now that i think about it... Rogal may not be the most mock-worthy but he definitely has the worst record.
I would say....Alpharius and/or Omegon. "Okay, so. Guys. This alien Cabal? They say if we help the guy who's trying to take over the Imperium we've spilled 200 years of blood over, actually do the deed, our Brother will kill our Dad, become host of Chaos itself, suddenly have a humongous guilt trip, and then spend about 50 years wiping out Humanity, to save the greater Galaxy from the taint of Chaos. Turns out we're like supercharged batteries for them. Okay? BREAK!" Ten Thousand Years Later "So, how's that Chaotic guilt trip comin'? Oh. You mean it's still not happened? Despite our help? So we're just stickin' with it, like, for realsies? Cuz, I'm gonna be honest...I'm starting to have doubts about it, you guys."
Angron=Daddy and Surgery Issues Fulgrim=The shining example of how much debauchery a space marine can succumb to Mortarion=Lazy SoB who can't even take ONE imperial palace *Grumble Grumble* Rowboat Girlyman=Control Freak Perturabo=Depressed loner who can't take a joke Sanguinius=Vampires, nuff said Alparhius, or Omegon, Alpharius Omegon=*Error, system data lost, cannot retrieve specified records* Lion El'Jonson=Hippy Jaghatai Khan=Mongol Wannabe Ferrus Manus=Suck up Leman Russ=Crazy Werewol-*GETS DOGGYPILED BY 300 SPACE WOLVES AND IS EATEN* Rogal Dorn=Gulliman Wannabe Magnus=Know it all... Lorgar=I dont like this guy...I dont know why I dont like him...I just dont... Vulkan=Cry baby (Also reminds me of a lava lamp) Corvus Corvax= Deviant 'lil brat... Horus=Does this really need an explanation? My work is done here.
With Fulgrim on the list, you're listing Guilliman under "debauchery"? I'd say Roboute Guilliman should be "control freak".
i have changed my mind. Leman Russ is most mock-worthy in my opinion. Reason below (Spoilers) Deliverance Lost (Yes, the book) informs us he dog DNA in his makeup. You could conceivably defeat him and his entire legion with Beggin' Strips, since they don't know its not bacon.
I feel bad for Omegon, i mean he should have stuck out like a sore thumb, what with the 20th all pretending to be Alpharius, it should kinda have been obvious. But anyway.