The awesome boss fights are just one of the reasons this hugely popular MMORPG has drawn in 11 million subscribers (the most of any game in the genre) and counting.
World of Warcraft has had several epic bosses, but one that stands out is Vaelastrasz. Yes, he was That One Boss to the point of shattering guilds without apology or remorse. Yes, those were the most balls-to-the-wall three minutes of your entire raiding career, even from the perspective of having done it all, from 40-man Naxx to Sunwell to Malygos. Intense does not begin to describe it.
"I am not some simple jester! I AM NIELAS ARAN!" This guy has what may be the best voice acting in the game. A terrified, savage and also dead mage that's totally freaked out while trying to kill you, plus making fun of mages when his mana gets too low? Awesome. Best boss ever even though he was That One Boss to the massive potential raid damage and crucial interrupt timers. He even made Karazhan cool.
Even more awesome if you (somehow) got Atieshnote He's Medivh's father and that's Medivh's staff you're carrying. The Dev Team Thinks of Everything. and had it equipped when you started the fight with him. Also, this.
Eldritch Abomination Herald Volazj is pretty popular due to his uniqueness: He sends the whole party insane, causing you to hallucinate that all your friends are trying to kill you, and you can only go back to killing him after they're all dead.
"PRIDE HERALDS THE END OF YOUR WORLD! COME, MORTALS, FACE THE WRATH OF THE SOULFLAYER." This is Hakkar, the Blood God, a fight where the only way to win is to literally poison your entire raid repeatedly.
C'thun, at 60, was a frantic fight where losing one raid member early could mean you didn't quite have the juice to keep up with his attacks, one of the first times in the game's history that every single life counted. And yet, the fight was ludicrously entertaining, to the point that people still go back 3 years and twenty levels later to experience it again. Notice the C'thun fight contains such originalities as an Eyebeam that jumps from player to player increasing in damage for every jump (can hit in the millions after a few hits, if not more) and swallowing players, taking them to his Stomach where they need to kill Flesh Tentacles to make his main body attackable. Also, he whispers creepy stuff to you.
Yogg-Saron in Wrath of the Lich King takes this to a whole new level, featuring all the epicness of C'thun and more (such as a Sanity Meter. If this runs out, the player will be mind controlled and see his entire party as servants of Yogg'saron.)
Yogg-Saron's voice acting is a big case of YMMV, and in particular, Yogg-Saron's laughter. In Phase 3, as you DPS Yogg himself, Yogg-Saron casts Lunatic Gaze. All players must turn their backs to Yogg when this happens because staring at him during Lunatic Gaze is staring into the maw of madness itself and destroys your Sanity (reducing it by 4% every second you look at him). To add to this, during Lunatic Gaze, Yogg-Saron laughs maniacally at your raid. This is either so awesome that it's actually terrifying or just Narm at its best/worst. The transition into Phase 2 features one of the best introduction emotes yet, complete with a case of Voice of the Legion between Sara and Yogg-Saron with Yogg's voice slowly overtaking Sara's voice until it's all Yogg's voice in the final line.
"I am the lucid dream. The monster in your nightmares. The fiend of a thousand faces. Cower before my true form. BOW DOWN BEFORE THE GOD OF DEATH!"
Arthas Menethil, the Lich King. Even leaving aside the fact that he's the current end-game boss, the fight itself is awesome. Six distinct phases, even if two are transition phases and the last is just beating up Arthas while he's unable to fight back. Everyone in the raid has to be on the ball and paying attention, as he is constantly throwing out abilities that will cause a wipe if they're not managed properly, such as Necrotic Plague (a disease that ticks for 50K a time and jumps to another target while getting more powerful if it kills what it's currently on), Infest (a DoT that does more damage per tick until everyone in the raid is healed past 90% health) and Defile (damages anyone standing on it, plus it gets bigger every time it deals damage until it potentially fills the entire area). He also, at different points during the fight, summons Val'kyr Guardians (to pick up a random person, carry them to the edge of the area and... just drop them) and Vile Spirits (which come in swarms and blow up when they get close enough). Then, during the third non-transition phase, he uses an ability called Harvest Soul, which sucks one person into Frostmourne where they have to help Arthas' father, Terenas II Menethil beat the Spirit Warden to escape. After all that, after he's reduced to 10% of his seventeen million starting health (actually he only has 17.4million in 10man regular, on 10man heroic he has 29.5million, on 25regular he has 63.1million and on 25man heroic he has ONE HUNDRED AND SEVEN MILLION HEALTH!!!)... that's when the true awesome begins. He then says, "You gnats actually hurt me! Perhaps I've toyed with you long enough, now taste the vengeance of the grave!" and unleashes the Fury of Frostmourne, doing one million damage to everyone in the raid. Next comes The Reveal that your progress through Icecrown Citadel, even quite possibly everything you'd done against the Scourge since your first arrival in Northrend, had been part of a Evil Plan by the Lich King to draw in Azeroth's greatest champions. In short, he needs new generals, since his previous ones are now scattered in small pieces throughout Icecrown Citadel. Oh, look - here are the 10/25 people that did it! He starts to resurrect the raid with the intent of making you his new generals, and this is when Tirion Fordring simply awesomes his way out of the ice block he'd been in since the fight started and shatters Frostmourne. The souls that had been imprisoned within the blade are freed, and grab Arthas. Terenas Menethil then resurrects the raid, and it's simply a matter of finishing Arthas off. Roll cinematic. The feeling of satisfaction that you get the first time you kill the Lich King? Priceless. Next stop: Icecrown Citadel Heroic modes!
Arthas is also noteworthy because his emotes are some of the best ever, especially his Hard Enrage (which can multiply the already staggering amount of awesome if you have one of those infamous scenarios where the enrage hits mere inches away from the deathblow). Combined with the great voice acting by Michael Mc Connohie, it's simply one of the greatest lines ever uttered. *ahem* FACE NOW YOUR TRAGIC END!!!!
Also, when he insta-kills the entire raid and the usual "Release spirit" button that players get upon death appears, if you try to click it a notification will appear saying "Cannot release! Your soul belongs to the Lich King!".
Arthas at the end of Halls of Reflection also counts. The music is impressive, and nicely fits the frantic struggle to kill the undead before he closes in and kills your group. It's quite an effective Final Boss Preview.
Flame Leviathan in Ulduar. If everyone knows how to use their vehicles, it's a fun battle against a giant tank with more HP than the Lich King that's a change of pace from other raid encounters without being too non-intuitive (shoot the boss and run away if he chases you). The reaction when Brann notices the Leviathan coming is a nice opening to the battle.
While not really tough, the boss Mordresh in the Bone Pile section of Razorfen Down is probably the most awesome moment in the lower leveled instances. Getting to fight a massive skeleton on top of a giant pile of bones leading a huge army of smaller skeletons at the bottom of the pile while dancing the undead mosh pit like dance and screaming how the Scourge will take over the Barrens.
Prince Malchezzar of Karazhan is definitely That One Boss and luck is a major factor in beating him, but he has some great quotes (especially "Surely you did not think you could win." as a Have a Nice Death), and taking the smug bastard down is so satisfying.
'Captain' Cookie, a low-level dungeon boss, is completely hilarious. He's a murloc who attacks from a cooking pot by throwing food at you. And he comes pretty much out of nowhere. It's great!
Tony Two-Tusks in Thousand Needles. His fight literally consists of him hopping in and out of treasure chests, while random other effects and mobs jump out of the others. Then when you finally kill him, his ghost follows you around making fun of you until you resolve the quest chain.
From the 85 entrance raid Blackwing Descent, we have Magmaw. An absolutely enormous magma worm in a giant lava pool that's chained to the floor you're on just so you can fight him. During the fight, he tries to swallow one of the tanks whole, in which he needs to be healed like no tomorrow. The two melee characters jump onto the boss and chain his head to the groundrevealing its far softer headthat takes double damage from attacks.
Kael'thas, final boss of Tempest Keep, is probably one of the most fun fights ever added in the game. First he pits you against his 4 advisors, one after another. Once you defeat them, you fight several legendary weapons; once you defeat them, you can loot them and use them in the fight, and they're far stronger than any weapon you have. After a time (back in the day, typically after most of the weapons were dead, but not all), he revives all 4 advisors and has them fight you at the same time. Once they die, you finally get to fight Kael'thas himself. Mind control, massive Pyroblast that needs to be interrupted before it one shots the tank, a shield to take down...a tough phase. And once you get his health down low enough, he removes gravity and has all the players flying through the air, avoiding sparks that do high damage. A very long, but extremely satisfying boss fight.
Lady Vashj in Serpentshrine Cavern, while definitely That One Boss, had some pretty cool mechanics; namely the Tainted Core. Phase 2 is an add fight where you have to kill certain adds and loot them for the Tainted Core, while other adds are roaming around and need to be destroyed. The player who loots the core gets rooted and can't move, and so he needs to throw the core to another player, who then gets paralyzed. The core gets tossed from player to player until someone uses it to break a shield generator. Do this four times, and you break her shield, letting you finally damage her while she summons poisonous creatures that will fill the room and kill you if you dont kill her fast enough. A very difficult fight to pull off, requiring extreme coordination from your entire raid group, but oh so satisfying when she finally dies.
Murozond, from heroic End Time. He's a fairly standard dragon boss, what makes him so much fun? There's a big hourglass that has five charges, and the players must periodically use it, which resets the player's status back to the beginning of the fight, while leaving Murozond at the same health. This is mainly used to clear the stacks of the debuff he throws on you, but it has the added effect of resetting all cooldowns. Basically, for this fight, you can ignore the cooldowns on your best buffs and abilities and use them to your heart's content. What makes this fight so awesome is that you can spend the fight in perma-Metamorphosis as Demo, perma-Tree Form as a healing Druid, have 100% uptime on Heroism with a Shaman or Mage in the group, and so much more. You can just go all out and it's a blast.
Even more insane is that, if timed correctly, some cooldowns will stack. Like a warlock having 2 or 3 Doomguards out at once.
The Mannoroth fight in heroic Well of Eternity. You're fighting alongside Illidan and Tyrande, which is totally badass, and you can pull some crazy high DPS near the end of the fight. Once you're overrun by demons, Illidan transforms the players into demons themselves, doing a massively strong Immolation Aura and letting you burn down the rest of Mannoroth's massive hp with little problem. It's a very easy fight, but the sheer epicness of fighting such a powerful boss (both HP and story-wise) makes up for it.
The Spine of Deathwing fight in the Dragon Soul raid. You are riding Deathwing - yes, the enormous dragon that broke the world - and you have to literally tear him apart by first nuking him with his own blood followed by cutting his own blazing tendons apart. And he's no idiot - he knows you're on top of him, and the second your party members go on one side, Deathwing starts to flip over, and you have to hang on to his magma-bleeding wounds for dear life.
Algalon the Observer, the optional super boss of Ulduar is definitely one of these, featuring stunning visual effects, epic attack names (how much grander can you get than Big Bang?) and a rare case of absolute stakes: he is less than an hour from informing the Titans that the world needs to be rebooted (erasing all life and starting over), and he even points out that defeating him won't stop the transmission, so the only option is to make him change his mind through a beating. Blizzard themselves specifically created him to be a massive challenge, requiring tight raid coordination, and even nicknamed him "Algalon the Raid Destroyer" themselves. He feeds on your tears.
The entirety of Siege of Orgrimmar can fall under this category since there is hardly a standard fight in there. Take the first boss. Giant water elemental that you have to not only kill parts of him when he splits, but also HEAL some of them.
Spoils of Pandaria is not an average boss fight. There is not that one big bad you fight, but crates full of them. Yes. Crates. Filled with enemies. And you have to break them open and kill them.
The Paragons of the Klaxxi in Siege of Orgrimmar. It's an intense and varied council style battle, but even more epic when you consider who you're fighting- nine of the greatest Mantid heroes over all the millennia of the mantid's existence. After defeating each of them, you can steal their powers and use them against them.
In the Trial of the Crusader, Grand Warlock Wilfred Fizzlebang tries to summon a doomguard but instead summons JARAXXUS, EREDAR LORD OF THE BURNING LEGION. He gets promptly blasted for his arrogance and the sheer awesomeness of this demon boss was too much to be contained within a single game - he is now also hamming it up in Hearthstone as well!