I don't get why people call it the best game of all time though. The fan base is horrible as well.
Calling it the best game of all time is a subjective sort of thing. it's the best game that
they personally have played.
And people call it that because they like the darn game, obviously. It's a game that focuses more on story than actual action, focuses on the emotional and meaning aspect rather than focusing on having fancy graphics (There's a distinction to be made between retro-style graphics and bad graphics; the graphics are well designed (in most cases), it's just pixelated.) or flashy combat. There are
very few games that have done it in the same fashion that Undertale has done. It's a game that's stood out as not just unique and different, but actually fun and good to like 98% of people.