S Tier
Chara, Asriel, and Flowey.
The emblematic of the games core message twins, I love these two, their mental illness is so intriguing. The min maxer who cares only for a game's utilities and the story brained perversely sentimental kid who's explored every possible route of his world. I think about these two on a daily basis.
I find the idea that the surface of the under tale being our world fascinating as well, especially because Asriel is turned into a flower exclusively found there. All of the former prince's whimsy for his fantasy land dying when he literally loses any ounce of magic within him? That goes incredibly hard.
Chara's unique sort of sociopathy representing a gamer's attitude towards a new game just makes my brain go apeshit, the little demon you have to convince this world is worth something. Personally, I think the red soul represents the concept of LOVE, that layered messy feeling, what do you LOVE about Undertale? Will you persist? If you don't care you won't.
A Tier
Alphys
Alphys. Anxious. Weeb. Liar. Lover. If you think she's annoying I think you have bad taste, admittedly... I sympathize with this character like 80% of the fanbase, a lot of the hate she gets online feels undeserved. I'm going to be real here, a good amount of said hate comes from people like the person who gave me the sapphick, when, they also complained about being reminded of queers when they see rainbows. That was a real internet interaction. Besides that there's some complaints I sort of understand, her internet posts and calls are cringe or something, it's on purpose, I guess what matters is if you find that endearing or not.
The True Lab plot is just *Mwah*.
Mettaton
Who is this diva? A TV show host, a narcissistic nutjob, attempted child murderer, and a trans masc icon. He's a very very entertaining character, I love him in the winter alarm clock stories. When it comes to presenation he's the king of the game, his designs, his themes, love this guy.
Mad Dummy and Mad Mew Mew
Character that turns you into a girl, hell man, post transition in like 2022 I was hyperfixated (no hyperbole) on the ghost cousins for the longest time. Coo coo kitty, she's fun :3.
Sans
Very well executed character, I don't think about him as much as other characters but he's good. I think why I don't personally care for him as much as I theoretically would is oversaturation, saw his face a little too much. He has some of the better joke lines out of the whole cast. I don't think I have much to add to the discussion around him.
Papyrus
He underestimates himself and overcompensates, it's hard to dislike him. He's one of the stronger characters, like, physically and emotionally stronger but he doesn't genuinely recognize that himself. The dynamic between him and his brother became so popular for a reason, they're more similar deep down than people think.
B Tier
Frisk
The more I understood Chara the less I understood Frisk, why are you here? Why are you built almost identically like the humans before you? Very mysterious, very mischievous, I don't know enough about them to place them higher on the list but they're cool.
Toriel
I like what she represents more than her herself, she's the Holy Spirit to Asgore's the Father and Asriel's the Son. She's our guide, who introduces us to this new world, Asriel's our journey through it, and Asgore is the man who we all meet at the end, who, in a sense, crafted the situation we're in. It's nice to have a mom in a video game who's more than symbol, we see how parenting affected and still affects her, she's well-meaning but dismissive.
catty's hotter
Asgore
Same deal here as well, well-meaning but flawed, in short-sighted rage he declared war on humans who fall into the Underground. He's a child murderer, nothing can quite justify that even if he didn't like doing it, not a fan of how he instantly becomes a joke in True Pacifist after the fight. I think his love for his own children and goofy side makes him likable to a degree but he was definitely a tyrant as lord of his kingdom.
Undyne
Fish warrior of the underground, the TRUE hero, the one the earth itself gave power to fight evil in its refusal to die, the woman who inspires those around her to keep on living. I find her a bit too one note sometimes on most routes but ultimately yet a character's who's concept is mostly well executed
Napstablook
Thinking again idk why I put them on this list lol, I'm sorry Blooky fans they don't do anything for me I'm afraid.