The Dark Souls series is home to several bosses designed with the intent to intimidate you and give you all your money's worth. While some are excruciatingly hard to beat, they still provide some of the most heated fights in a video game of this genre. Unmarked spoilers ahead.
The Bell Gargoyles. After the trek through the Undead Burg has tempered you into expecting what Dark Souls has to offer, you find yourself in the Undead Parish, where the first of two Bells of Awakening await you. The final obstacle is a towering gargoyle swinging both its tail axe and its halberd to smash you into the roof you fight upon. And then, when you've whittled down its health to half, another one joins the fray, blasting fire at you as you now deal with two opponents. Bonus points for this being the first point you can summon Solaire in the game.
The Iron Golem. You've been through hell and back after finishing the gauntlet of traps that is Sen's Fortress, and this colossus is the only enemy left that guards the way to Anor Londo. Your train of thought? Bring It! Bonus points for being allowed to summon the almightyBlackIronTarkus.
Gwyn, Lord of Cinder. While the buildup prior to encountering him was dreary and foreboding, you enter the arena and are treated to a melancholic music piece. After learning of the circumstances behind this sudden change in music, you realize that Gwyn still clings on desperately to a long-forgotten desire to preserve the Age of Fire, and happily gives you one of the toughest duels in the game. The fight gets even better if you managed to save Solaire, allowing you to summon him and beat Gwyn together in jolly cooperation.
The Sanctuary Guardian. A winged manticore that shoots lightning and punishes you at the smallest mistake. You also fight it in an open area, leaving you with only your raw skills to overcome this fearsome beast, which becomes immensely satisfying once you do so. Want to get full access to the additional content? Defeat it first!
Knight Artorias. A clear-cut expy of Guts, this guy not only gives you one hell of a boss introduction, but is also a bonafide Lightning Bruiser on par with Gwyn, if not better. Between the crazy spin attacks and sword-plunging somersaults, Artorias lives up to his reputation as one of the most praised characters in the Dark Souls lore.
Manus, Father of the Abyss. See Artorias above? This bad guy proceeds to up the ante and attack you relentlessly from the start, overwhelming you with heavy flurries and shadow sorceries. Not all hope is lost, however, for you are able to summon Great Grey Wolf Sif for the fight, giving you the courage to put a stop to this omnicidal abomination for good.
The Pursuer, a massive levitating armoured knight who appears halfway through the first level...outside his boss arena. That's right, he doesn't wait for you to come to him, he comes to you instead!
The Last Giant, a foe so huge you can only target his feet and ankles. His opening cinematic has him tear himself free of the rubble confining him in a particularly badass way, and once you reduce his health enough he'll rip off his own arm and use it as a freaking club.
The Old Dragonslayer, a shadow of the previous game's Ornstein. As soon as you enter the Cathedral of Blue, you're immediately confronted by this familiar face, forcing you to fight once again amidst breakable pillars, avoiding his powerful spear thrusts. This is topped by his unique Leitmotif, "The Old Dragonslayer". Many players report seeing Ornstein again and panicking, spinning around wildly looking for Smough.
The Lost Sinner, a nameless undead warrior entombed in Sinner's Rise. Having her hands shackled together does nothing to stop her from cleaving you in half with her ultra-greatsword. Unlike some of Dark Souls II's enemies The Lost Sinner has no gimmicks at all, simply being an ultra-agressive swordswoman yet still managing to be one of the most memorable bosses in the game.
The Duke's Dear Freja, apparently borne out of the Duke's fascination with arachnids and the like. While the boss itself is fairly straightforward, the really impressive part comes after beating it... no Great Soul drops. It's sitting in the open at the back of the arena, directly below a cocooned dragon - meaning the player had to kill something tougher to get it.
The Looking Glass Knight, where the game drops all pretences of subtlety and INDULGES in a gorgeous display of graphic prowess with an unforgettable Battle in the Rain. From the boss' intimidating presence to the bolts of lightning he unleashes, there's nothing quite as fun as taking him down in the middle of a storm. Oh, and his mirror shield can summon NPCs or even other players, akin to the Old Monk.
The first Guardian Dragon. Ever wondered how a straight-up boss fight against Dark Souls's Hellkite Dragon would turn out? This is it. After the bone-chilling trip in Aldia's Keep, you finally get to fight one of the most prominent creatures featured in the pre-launch trailers (right up there with the Looking Glass Knight). Despite fighting it in a massive cage, this wyvern still gets to fly around a bit, and makes full use of the boss room, all while hailing you with a trail of fire breath. Bonus points for the glorious Scenery Porn that comes after the boss fight.
The Darklurker. Just reaching it is a challenge (requires finding and joining and obscure covenant and fighting through several gauntlets of black phantoms). The actual boss fight pits you against an eerie angelic figure who is bar none the most powerful spell-caster in the game. The entire battle is a desperate race to whittle down its (thankfully relatively low) health any way you can while it hurls pyromancies, hexes, and sorceries at you. Then, just when you think you've got it on the ropes, it duplicates itself.
The Crown of the Sunken King DLC adds Sinh, the Slumbering Dragon. A magnificent reprisal of Kalameet's legendary boss battle, Sinh flies around while raining down fire on you, before landing in a dramatic fashion and engaging you in a ground fight, complete with toxic-infused fireballs and environmental damage.