1. Firelands and MC Ragnaros; equally amazing.
2. Lei Shen: I love how the platform changes relative to what's going on in the fight.
3. Algalon: I've never actually killed this boss but it looks so awesome in the videos.
Honorable Mentions: Primordius, Mimiron, Maexxna, Thaddius, Kil'jaeden, Elegon