Kroq-gar on grymlok or
Kholek
GOJIRA :D
I'd like to think kroq-gar, he's been at it since the first chaos incursion so he's bound to be a good fighter. Not sure on kholeks credentials, is he just really big with a big lightning hammer?
Kholek was there as well when the pact was made between the Dragon Ogre race and the Chaos Gods.
So he is immensely old too.
I'd say Kholek, considering his huge size and Starcrusher.
Kholek is older but Kroq-Gar probably had more combat experience. Dragon Ogres like full dragons like sleeping, so Kholek hasn't been constantly battling his whole life like Kroq-Gar has. In lore Kholek is much bigger than Grymlok so he would have a physical advantage. I think Kroq's best chance would be to have Grymloq get him near Kholeks head where Kroq could get a fatal blow on Kholeks head. If Kholek manages to kill Grymlok before Kroq can get close then Kroq doesn't stand much of a chance. That's just my take on the matter.