I should probably asked "What would your ultimate raid be" instead but let's just run with it. For some people, Icecrown Citadel is the raid and the Lich King is the encounter they've been waiting for since WoW shipped. Arthas/The Lich King is on a lot of players' lists of favorite bad guy. For me, personally, there are a few contenders for 'Ultimate' raid status, as defined as both super-epic in lore and scope and the older definition of ultimate as the end. For me, if I ever get my ultimate raid, I may stop playing WoW.
- Deathwing. Frankly, if they do this fight the justice the character deserves, the fight will be so awesome that it may destroy my WoW installation upon finishing it. Deathwing is a character I've wanted to see more done with since our old days raiding BWL. I want cinematics, unique models, unique music, new mechanics, the works. I'm actually somewhat nervous about this character and hope his entrance in Cataclysm feels as portentous as it should.
- Sargeras. We've yet to actually see Sargeras himself in the game. His spirit is supposedly drifting in the Twisting Nether, unable to access his original form, so will we eventually end up fighting him twice? Will we have to try and stop him from reclaiming his original titanic form and power, and then fight him later once he does? And will we even be on Azeroth when the confrontation takes place, since for him to enter our world bodily seems to take a massive amount of power (see the original Well of Eternity)?
- Azshara. I really dig the Naga and I want to see more done with them. I know we'll have underwater cities and such in Cataclysm, but I'm eager to get to see the main event, so to speak. And Azshara ties well into the whole Sundering/Well of Eternity legacy which would make for all sorts of story hooks. I'm eager to see this character make an onscreen appearance in WoW.
- Someone new entirely. Old gods and established lore characters are great, but one of the things that Vanilla WoW did was provide us with new or expanded enemies to fight and I'd like to see more of that. Nefarian and Onyxia, Ragnaros, even C'Thun were all new to World of Warcraft itself (although the Black Dragonflight and Old Gods weren't.) There's no reason something else from the Twisted Nether or Great Dark Beyond couldn't menace Azeroth, or some mortal wizard or warrior couldn't acquire vast power and become a threat. The Lich King had ties to the previous two Warcraft games but was new at the same time. Let's see something like that happen again and an entirely new menace confront us.
- The Gronn. I kind of feel like the Gronn got short shrift in Burning Crusade. Here are these gigantic, semi-immortal beings of immense size and power, strong enough to go toe to toe with potent dragons and led by Gruul, who mysteriously has seven 'sons' (I can't tell Gronn genders, man, for all I know they use parthenogenesis to reproduce) each nearly as massive and vicious as he is. In Beyond the Dark Portal, Gruul is supposedly smart enough to know how to use the Alliance to help him defeat Deathwing, but in the game he's just kind of a drooling idiot. I'd enjoy a raid that dealt with the true origins of the ogres, their relation to the Gronn, and where the Gronn lords and Gruul actually came from.