Which of the skellies do you think would be happiest with an S/O that p much constantly showers them with affection? Compliments them, says they make the S/O happy, listens attentively to what they say, gives them lots of hugs, kisses, little touches on the shoulders, back, hips, lifts them up and spins them around in joy, nuzzles them, gives them massages, etc etc? Kind of making it impossible not to notice that the S/O loves the skelly (though they'll cool it when/if the skelly wants them to).
Undertale Papyrus - PLEASE MARRY HIM. It's literally all he asks for. Papyrus is extremely needy in affection and he is always around you somewhere. He is so happy that his soul is shining like the sun in middle of summer. It's also makes Sans very happy to see his brother that happy. Everyone is so happy even the rainbows are throwing up other rainbows.
Underswap Papyrus - He's dead. You killed him. He's not use having that much of affection, but god, he is in paradise now. Most of the time, he turned entirely orange whenever he sees you because he knows what's coming. He loves how you are taking care of him, but he is also very shy, so you'll have to guess what he feels while he's mumbling things that you can't understand.
Underfell Sans - Ok. So, he pretends he doesn't like this, because, you know, he is a tough guy. But that small purring he is desesperately trying to hide is not fooling you. + He gets angry when you are leaving the house without cuddling him now. Before realising why he is angry and getting entirely red.
Horrortale Sans - He is purring 24/24 h. You can't stop him. He can't stop either. He's a big cuddly boy and you give him the attention he needs, there's nothing you can do to make him happier. Willow has to keep you two away sometimes because Oak's purr is VERY loud and it can be a bit tiring to hear it all the time. It's okay, you can go outside. He loves going for a walk anyway. You can also say he is really happy because he doesn't feel the need to run away anymore. All he wants is to cuddle on the sofa.
Horrorfell Papyrus - Ok, Chief hates when people are too touchy with him, because the few people who showed him affection Underground tried to stab him the next second. But now you are always cuddling and touching and petting him and he is so confused ??? What are you doing ? Are you searching for a weak point ? When you feel it's too much, you are leaving him alone, but he is always looking at you afterwards like "who said you could stop ?". Ok, maybe Chief is liking that.
Dancefell Papyrus - He is a diva, of course he needs someone to comfort him while he is complaining about how hard his life is. Or maybe that's just an excuse because he knows you'll give him affection when he's doing that. Who knows.
Disbelief Papyrus - He's just like Papyrus, but he is a little more shyer and won't ask for affection because he thinks he is too much. But oh god, just pet his head and he is all yours. He is even more needy than Papyrus since you are all he has.
Dust Sans - You are petting his head and he is looking at you without blinking. So you stops, wondering if maybe he's not liking that. Then he takes your hand and put it on his head. "Again." He can spend hours doing that. You are not sure what to think about this.