Definitely as OvidsExile said, Juan Francisco Estrada without a doubt (although he could be slipping now).
Shinsuke Yamanaka. He's been defending his title for 6 years with wins over Aneselmo Moreno, Vic Darchinyan, and some other very solid fighters, and yet never a whisper about him. Yamanaka is one of the best fighters in the world. He's also fighting this weekend, going for his 12th defense of his bantamweight title. It's just too bad he's getting up there in ring years.
Hard to say. He doesn't fight enough. He beat up Willie Nelson in June of 2016. Before that he fought once in 2015, once in 2014. His last meaningful fight was 4 years ago against Vanes getting a SD Kind of hard to underrate a fighter when he's so inactive
I will cop some flak for this but I always thought this forum underrated Miguel Cotto. Obviously this is localised to this forum through my lens, not the wider world
Another underrated boxer In my opinion is luke Campbell in the lightweight division, he's fast, got power and good ring work, suffered 1 bad loss but can easily win a title still.