The true final boss of Cataclysm's first tier is a hotly debated topic. With three different raid instances available at launch, the end boss in each raid zone could be eligible for the top spot. While most people will agree that Al'Akir is only a secondary opponent, Nefarian and Cho'gall are both old and powerful enemies. Cho'gall has been covered in the WoW comic books with great detail, while Nefarian is so common in WoW that we've already killed him once. Sinestra could even make her claim for the throne, as she is a difficult, heroic-only boss.
In my opinion, Nefarian is the final boss of tier 11. He's Deathwing's son, which makes him public enemy #2. During the encounter, we're faced with not one but two dragons, plus all the doomfire and adds that we can handle. The Nefarian encounter is the type of fight that I would've hated before Cataclysm. Now, we have AoE abilities to handle the incredible raid damage and the mana management tools to keep ourselves from running dry.