Fighting all of these awesome Undertale bosses will fill you with determination.
Since this is a list of boss battles, spoilers are to be expected. All spoilers are unmarked!
- Papyrus is a pretty awesome introduction to the types of bosses the game will throw at you. It's a very humorous boss fight that seems easy at first, even when he uses his telegraphed "Blue Attack"... but then your soul/heart also turns blue, forcing you to jump to avoid his attacks, which makes the fight harder, yet also makes it more fun and exciting, showing what kinds of curveballs the game might throw at you.
- Mad Dummy. His battle style is different from anything else in the game. Instead of fighting or sparing him, you need to lead the attacks of his own Dummy Bots into him. The dialogue is nothing to sneeze at either, with his Large Ham tendencies, his firing all of his minions for their ineptitude, and him being based on one of the most popular and effective villains in the history of manga. To top it all off, the music is a really cool remix of an earlier theme.
- Undyne's fight is just as amazing. Her first move is to turn your soul green, which prevents you from escaping but also gives your soul a kind of shield. This shield allows you to deflect her attacks — which come at you from all sides, by the way. They also come at various speeds: at one point, you're faced with an extremely slow moving attack coming at you from one side, only for several quick attacks to come at you from the other side. Eventually, she adds attacks that flip around to your exposed back, requiring you to spin around quickly to block them. Near the end of the encounter, she throws a lot of these attacks at you, making you spin around rapidly trying to block them all. In some rare in-battle dialogue, the entire act of Undyne turning your soul green is stated to be her handing you another spear to fight her fairly. Picturing the fight as a sparring match between Frisk and Undyne makes the battle a whole other layer of awesome.
- Muffet's battle takes Papyrus's battle's reason for being awesome and takes it Up to Eleven. First, she throws her movement curveball—having to move along and jump between three tracks to dodge waves of spiders, donuts, and croissants—on her first turn. Then she curveballs that by making the bullet box move while still throwing spiders at you, which in turn precedes her sending her pet to chase you, requiring you to climb up the webs to avoid it while still dodging spiders. While frustrating for first time players, the unique mechanics make it one of the most complex and thrilling battles in the game. The upbeat battle music is the icing on the cake.
- Mettaton EX's battle starts with him revealing that the entire 'killer robot' routine was a hoax Alphys created with Mettaton's help to make Alphys look good. In fact, Mettaton doesn't want to kill humans; it's his lifelong dream to perform for them. Mettaton tells the player that Asgore will almost definitely beat them and proceed to destroy humanity, and he simply cannot let that happen. The fight includes (in no particular order): an epic Transformation Sequence, unique sparing mechanics, bombs being shot out the sky, pop quizzes, viewer call-ins, a smoke machine, lots of poses, tiny robots floating in and attacking the player with hearts, and Mettaton's legs. All watched by the entire underground on live television. It ends with Mettaton realizing, after the aforementioned viewer call-ins, how much he means to his fans in the underground, and how much they mean to him. Still not convinced? The battle theme is called 'Death By Glamour'.
- Asgore Dreemurr, King of all monsters. One of the few bosses you absolutely cannot spare and must defeat by force. The boss begins by destroying the MERCY button! The battle has no sudden tricks to throw you off or anything you haven't faced before — it's a pure test of your skill at dodging bullet patterns and reacting to blue and orange attacks. Even using tricks to make the battle easier, the fight is still pretty long and difficult.
- The Final Boss of the Neutral Route, Photoshop Flowey, is an amazing fight. The buildup for it is great, as when Flowey gets the souls that Asgore gathered, the game window closes. Upon booting the game back up, the opening sequence glitches out and your save file has been seemingly hijacked by Flowey. Selecting this file leads to a terrifying battle with the power-mad flower, who has turned into a strange but terrfying Eldritch Abomination comprising photo-realistic faces, mechanical parts, and plants. You don't even get to use your menu commands at first. Instead, you simply have to survive, dodging several bizarre and erratic attacks. To cap it off, he can save state during the fight, which he can later load to catch you off guard. The whole fight in all has been compared to Giygas in its unexpected creepiness.
- Asriel Dreemurr, from the Pacifist route. Visual Effects of Awesome plus a healthy dose of Awesome Music with "Hopes and Dreams" is one thing. But the fact that it's an inverse of a Hopeless Boss Fight is worked into the story; you can't lose, because your determination to save your friends and the world is strong enough to endure the pain his attacks deal to your SOUL. Even when you can't move, and can't SAVE yourself anymore, then maybe you can save your friends. You enter Asriel's SOUL, save your friends, and even save the boss. Your determination brought you a long way.
- For the opposite reasons, both of the bosses exclusive to the No Mercy route, Undyne the Undying and Sans. Respectively, Undyne, basically souped up in one of the most awe-inspiring moments in the entire game and given dialogue that calls out to pretty much every climactic moment where the hero wins by pure willpower (the difference here is that you're a Villain Protagonist), as well as being given one of the best music tracks in the game, and a boss who's fully aware of your save game antics, combining awesome music and a hectic, incredibly difficult fight into a thrilling battle, featuring a combination of new tricks and the return of the "blue attack" in a way that will make experienced players remember what they did to Papyrus. Made even better when he turns your own tricks against you or refuses to end his turn so you can't have yours. They're considered two of the hardest bosses in the game in a route where anyone playing knows damn well by now both how much they deserve harder fights than what the rest offer, and that this is exactly what they wanted. For some people, in fact, throwing themselves at these two bosses is the entire point of running No Mercy.
- The Final Boss of No Mercy, Sans, deserves special mention as he is brutally difficult not by being powerful, but by turning the conventional ideas of Turn-Based RPGs onto their heads. Bosses use their ultimate / special attack near the end of the fight? Hell no, he opens the battle with his strongest attack right away. Bosses have a Battle Intro that they will say every time they're about to fight? He'll skip his intro to sucker-punch you. You have 99 ATK and DEF while he has 1 ATK and 1 DEF? He'll mock you for thinking numbers equal power by simply sidestepping your overkill strikes and delivering a Death of a Thousand Cuts that bypasses your DEF and invincibility frames. Turn-based RPGs are supposed to be Turn-Based? He'll attack you while you browse your menu, during your turn. You start getting used to his attack pattern? He'll start using Smash Cut to trick, misdirect, and confuse you. And the best part? He knows of your Save/Load antics, and every time you die against him, he will rub it in your face.
- The credits sequence in the pacifist ending. One of the most hectic bullet hell sequences in the game (trying to avoid all the names is challenging but fair), set to a fittingly amazing music track.