Why is it great?
The battle system is unique and can vary based on playstyle/story path.
Story paths and decisions are actually different and matter. For example if you choose to go the neutral route, and then instead of going true pacificist you go for genocide, there certain characters that make you feel bad for it. Every decision you make has consequences, and they pull your emotional strings.
The story is good, and the characters are well developed. The backstory is also nice.
The characters have charm and are humourous. They are also very unique as far as video game characters go.
The world feels immersive, and is well-thought out. There are no plotholes.
The music is awesome, and the aesthetics while simple, are nice to look at.
Gameplay and story are intertwined well.
The world is interesting and fun to explore.
Those are just a few things that come to mind.
