A tough question. It's hard to single out one boss, so I'll give a list of favorites instead.

Odyn - A shocking weakness! I like both of his fights.He's got cool voice and fun mechanics. Also, huge ballz.

Kael'thas Sunstrider - Tempest Keep was merely a setback! Both his fights were fun and full of lore, being turning points in wow's timeline.

Mimiron - fantastic mechanics, lots of fun, unique boss.

Postmaster Malown - personal bias here, but I loved spawning this guy back in vanilla. Also "You just got MALOWNED!"

Opera - 3+3 events rolled into one.

Viz'aduum the Watcher - fantastic mechanics and epic.

Honorable mentions:

Kologarn - Silly ass giant statue with regenerating limbs, aw yeah!

General Vezax - fun fight at its time with zero mana regen.

Moroes - Both versions, fun fight with fairly unique CC strategy. The old one was also interesting to cheese out of garrote.

Murmur - the epitome of heroic dungeon end boss at it's time, fun, unique and unforgiving.

Darkmaster Gandling - The old boss was amazing for teleporting you into different rooms.

The lurker below and Gahz'ranka - just love fishing bosses

Favourite Quest Boss: Darkreaver, a Death Knight that you had to slay in order to gain your Charger Mount back in the day. The entire quest was painful, drained my wallet completely and gave me PTSD for years after, but once I killed that bastard and got my 100% Mount, it was bloody worth it.

Favourite Dungeon Boss: Warchief Kargath Bladefist. Mainly due to my TBC nostalgia, but also because The Shattered Halls was hard. It was the testing ground for Raiders back in the day, and my fondest memory of any TBC dungeon was not only completing my Shattered Halls HC Raid Trial without dying, but healing through the hardest Dungeon Boss I'd ever seen and kicking off my healing career with a bang!

Favourite Raid Boss: Kael'thas Sunstrider, easily. Entering the room at the time was so epic-- it gave this horrible sense of royal foreboding, almost like we were meant to kneel before attempting to kill the bastard, and to get to him, we even had to go through his council! Not only were some of his minions annoying, Telonicus and his damn toys to name just one, but it was the first endboss fight that felt like a journey in of itself. The voice acting was incredible, the raid mechanics and faux-legendaries were incredible and the fight as a whole has just never been beaten out for that top spot, in my opinion.

Honourable Mentions:

Nightbane:- He was the first Raid Boss I ever killed, and even though the Guild Master's jealousy and the overall unpleasantness that followed made the aftermath rather painful, the memory of killing him and the sense of accomplishment I felt was what made me fall in love with raiding.

The Lurker Below:- A boss that you have to fish up? Perfect. Nothing more to say.