Number 1 - Jaraxxus
This guy is fucking awesome. He bursts in, kills a gnome and then proceeds to summon a shitload of demons in front of the audience of the Argent Tournament. He is one my favorite fights in the entire game and his voice sounds so fucking evil and yet so cliché at the same time. It's a great fight, a great boss and a nice guy who always reminds you who you're facing. That's right: "You face Jaraxxus!"
Number 2 - Malygos
I don't think Malygos as a raid encounter was well executed in the game. You met him once outside of the raid instance but many people still had no fucking idea who he was. And that's a shame when you're dealing with a dragon aspect. When I heard that Malygos was gonna be a raid encounter I figured he'd be inside the Nexus, guarded by a shitload of blue dragons. But no.
He was a fucking one-man raid encounter and his defense was so weak that everyone could just step inside his raid and face him. No need to kill a few guards or a few bosses before him. Just Malygos. That sucks. I think Malygos deserves more than just that - perhaps we could even be joined by Alexstrasza to kill him. All we got was a lousy, piss-easy vehicle fight.
Number 3 - Drakuru
Okay, this guy is a boss for a quest but still - he's fucking brilliant. He is a troll deathknight and maintains his troll accent even in undeath which is hilarious. That and he is complete and utterly evil! He works for the Lich King and tricks the player into helping him. He is a manipulative bastard with a cool accent. We players finished him off in Zul'Drak.
I have a dream that one day Drakuru will return as a raid boss in the yet-to-be-announced Gundrak raid. He could be resurrected by a bunch of necromancers or something, I don't care, I just wanna fight him again because he's so awesome!
Number 4 - Argent Confessor Paletress
Have you heard her? Have you seen her!? If you haven't seen her, take a look at this:
Imagine fighting a girl dressed like that and upon landing a strike on her, hearing her scream as if she has a fucking orgasm! That alone qualifies her for this list. More orgasm bosses, Blizzard, please!
Number 5 - Nefarian
Nefarian was a pretty unique fight because he called out for different classes which had unique downsides for each class. It was insanely fun and a great fight. Luckily he actually returns in Cataclysm, so I won't write much about him. All that's left to say is: Let the games begin!
Nefarian fucking rules.