All of mine are more "successful within certain groups of people, particularly internet communities" more than mainstream media, but:
The story premise itself wasn't THAT weird, but the worldbuilding and characters in Undertale were pretty out there.
Also, in relation, Homestuck was a wild ride from start to finish and was wildly popular for quite a few years.
Finally, honorable mention to intentionally weird stuff like Don't Hug Me I'm Scared and OFF that was mostly just popular on like, 2010's Tumblr for a bit, but still had their fifteen minutes of fame.