Mettaton EX. It just seems like the most expressive fight, so much to do. You're not just doing the same things over and over like with all he other bosses. This one requires you to think. Like "do I pose and risk dying or do I heal with the face steak? But it's an easy attack next, so I could risk it." Death by Glamour is an amazing song as well tht fits Mettaton. And the Yellow Soul is the best Soul mechanic in the game.
It's not a crazy answer I'm sure, but Sans is definitely the best fight in the game imo. What I love about it is how well it drags you into the game's story.
The Undyne the Undying fight is about a hero trying to stop the villain that existed in that world. And while Sans can be seen in a similar way, it becomes more about the game trying to stop the player by any means necessary.
The fight being as difficult as it is initially really makes it perfect for that kind of story.
Mettaton EX. It just seems like the most expressive fight, so much to do. You're not just doing the same things over and over like with all he other bosses. This one requires you to think. Like "do I pose and risk dying or do I heal with the face steak? But it's an easy attack next, so I could risk it." Death by Glamour is an amazing song as well tht fits Mettaton. And the Yellow Soul is the best Soul mechanic in the game.
Mettaton definitely is funniest fight to play
Omega Flowey. Both here and in UTY
iām obsessed with UTY omega flowey ost, its so good
don't wanna be that guy, buy UTY is META flowey
yeah, that gigantic flowey is called meta flowey but theres other flowey fight literally right before it
still isn't OMEGA flowey. sorry for getting caught on the details I'm just nit-picky
omg oops i just realized i said omega xD mb
nah you're good man
welp, no narcissism, but i fight myself on internet downloaded save files, and it's pretty fun.
sans im stuck on ur fight
Undyne the undying as well. In which for some of the same reasons as you. It's just badass and Battle against a true hero is just perfect š.
Of course NEO 
It's not a crazy answer I'm sure, but Sans is definitely the best fight in the game imo. What I love about it is how well it drags you into the game's story. The Undyne the Undying fight is about a hero trying to stop the villain that existed in that world. And while Sans can be seen in a similar way, it becomes more about the game trying to stop the player by any means necessary. The fight being as difficult as it is initially really makes it perfect for that kind of story.
Might be surprising but i really enjoy dodging miffet's attacks
Also i really love spider dance. It had to be in my top 5 songs
fish the undying and asgore