I really liked Warboss Grimskull from Space Marine. His feats may not rival those of the other warbosses, but since the very beginning of the game you and he share a rivalry that persists until the up close and personal finale. The interactions with him as well as the atmosphere of the boss fight by itself (the dark room created a feeling that you were being hounded by a monster, especially after being separated from your squad without any contact) really were memorable for me.
Wazdakka Gutsmek and his galaxy-wide Waaagh. Since Kaptin Badrukk and Boss Zagstruk aren't on here for some reason.
Wazdakka ain't dead yet! We orks gotta believe! But seriously, Wazdakka is still in the lore, still alive and kicking. Just no rules for him.
I note that there is an absence of Kaptin Bluddflagg in there. The freebootaz got dere own WAAAGH!s going, ya gits. (Mind you he doesn't actually have tabletop rules, but he's arguably one of the coolest things to come out of the Dawn of War series since Eliphas and Gorgutz)
Bluddflagg is cool and all that, but does he have his own band of flash gitz? Well, maybe he does, but I bet Kap'n Badrukk has ten times as many flash gitz as Bluddflagg does! But I suppose Bluddflagg is better overall than Badrukk. While Badrukk offers his services in return for loot and dakka, Bluddflagg conquers whole star systems, and armies of the Imperium, and a Daemon Prince, all for a hat. Mind you, it was a really good hat! Also, I heard that the Imperial Guard won the Kaurava campaign. Do you really think anyone would let orks have a cannon victory?
