Excellent replies, everyone! Everyone did a good job in picking shows that are somewhat obscure and underappreciated even by the "hardcore fan" standards of Anime Suki and RRRound Table.
I'm split on if
Flip Flappers should count or not, though. While
Tempester is right about it being very popular among a certain range of anime fans, Flip Flappers also ended up bombing sales-wise, sadly.
So I certainly feel like it's underappreciated, at the very least. Perhaps underrated as well.