Dark Lin-- oh wait that would be incredibly cliché...
I'd have to say Ghoma, more specifically the Twilight Princess incarnation. Just the idea of controlling a giant mechanical spider seems pretty damn cool to me. You have the ability to crush enemies by simply walking over them, you have the ability to climb vertically and even lurk upside down to surprise-attack your enemies, you can call upon mass army of baby ghomas to help you, and shoot giant laser beams.
Hyrule Warriors seems different to the typical Zelda in the sense that it looks like it's making you feel incredibly powerful in combat. This empowerment is exactly the reason why I chose Ghoma (Armoghoma). Imagine the destruction you could cause. Also, to avoid making it too OP, you could possibly make it exactly like how the actual boss worked in that, if enemies his the weak spot, it causes significant damage. This would make using Ghoma more tactical as you're attacks are extremely powerful, and you're resistant to a lot of attacks; however of enemies hit the weak spot, it heavily increases the chance of death.
Damn I wished Nintendo included bosses (excluding boss villains) as playable characters now.