That would be the old gods in general. So far there are three known of, C'thun, Yogg Saron, and a nameless one impaled by a MASSIVE spear. C'thun was taken on in the AQ40 raid back in the vanilla game, and Yogg-Saron has been reimprisoned in Ulduar in Wrath.
We (the players) haven't actually killed these guys, just sent them into remission if you will. C'thun is being revived for the expansion (I believe) and Yogg-Saron currently has the norse pantheon guarding it, now that we freed them from the crazy that Loken (Loki) hit them with.
Anub-arak got a bit shafted in the expansion. He appeared "alive" in a 5 man dungeon. Meaning that he was literally killed over a million times during dungeon running. The fanbase yelled because it wasn't doing him justice, so the developers brought him back to life for the Trial of the Crusader raid as the final (technically "surprise") boss.
There is nobody in Icecrown Citadel who a Warcraft player would know (I mean the strat games) beyond Arthas (I think there shoudl have been other guys in there that were with the King for a long time). Arthas is officially dead (mainly due to the souls within frostmourne rebelling against him). The way he died was... spectacular in showing the players that we are just insignificant gnats compared to him (as part of the fight, when he hits 10% life he waves his hand and kills your raid. Terenas, his father, appears out of Frostmourne, and brings the raid back to life and uses the souls in the sword out to hold Arthas while you beat him into the floor.)
Although, by far the hardest boss fight was with Algalon the Observer. Admittedly he became easier with gear, but Algalon and Yogg-Saron are two of the hardest fights in the game currently (barring Arthas and Sindragosa hardmodes). Algalon is actually a Titan... In the ulduar story arc, you learn about Yogg Saron breaking his prison and subverting his guardians (mainly loken). As part of the 5 man arc (meaning before the actual raid) you enter the Halls of Lightning and fight Loken, killing him (of course). When he died however, a signal was triggered by Titan safeguards that informed them something was very.. very wrong (Loken had frozen the signal about Yogg's prison). They sent Algalon to investigate and determine if Azeroth should be rebuilt (meaning all life would be eliminated, and the entire planet sundered to start anew). So (of course) the raid has to fight back against Algalon, you actually "summon" him (basically they made it so you couldn't take him on until you were good enough to try) and get exactly one hour from the time you summon him to beat him, otherwise he departs to inform the titans that Azeroth needs to be reordered. For Lore sake, they say we beat him (very few actually pulled it off when it was "current") and apparently the denizens of Azeroth are the first to fight back, which causes a bit of an existential quandary for Algalon
Basically he feels an emotion for once, and then gives you a code that'll prevent the other titans from rebuilding the planet while he wonders off to ponder this.
...
Why I just went on that tangent I don't know.