Fantasy, I'd say any Ogre model except Yhetees (Need to be more Ogre-like) and... *Shudder* Gorgers. TERRIBLE models because of the far-too-big-to-be-real heads. But the bulls, Tyrant, Greasus, Leadbelchers, Ironguts and the Ninja Maneater, especially the Ninja Maneater, make me want to start an Ogres army.
40k, well the new air-guitaring Daemon Prince looks pretty epic, but I might have to go for either Typhus, Huron, Jain Zar (Who looks like she's trying to score a conversion kick) or Maugan Ra, whom I recently painted for a friend and thoroughly enjoyed.
EDIT: Having just looked on the FW site, I change my 40k nominations to the Ogryn Berserkers. All of them are just stunning.