Sans is awesome! He is one of my favorites.He is deeply depressed feeling like his life is meaningless (with good reason ) and uses humor to cope. Alphys well she is the embodiment of nerdiness but she also made some terrible mistakes that she has to grow up and face. That is something we all have to do to some degree. So for those reasons I like her.
Mettaton is my other favorite though funny guy egotistical and thinking what he needed was out there somewhere when he impacts so many around him but doesn't see it.