I have a top 15 of professionals which i consider a great one for this category.I consider my top 15 which includes the most underrated or UNKNOWN TO THE MEDIA champions, the best i could think at work wiithout using a book or help of any kind, but i am sure some people can find more underrated champions than mine,i would too if i was was at home lol!Still i doubt the average (Tyson vs Ali and Pacman vs Mayweather) boxing fan here could even name any of my top 15! lol
My top 15 in no specific order

Juan Martin Coggi (Argentina)
Patrizio Oliva (Italy)
Johnny Bredahl (Denmark)
Marcel Thil- Probably the most underrated of all (France)
Rocky Mattioli (Australian-Italy)
Sung-Kil Moon (South Korea)
Anaclet Wamba (France)
Leonard Doroftei (Romania)
Vic Toweel (South Africa)
Pedro Carrasco -Probably second most underrated ever with a record 111-3-2 (Spain)
Mate Parlov (Yugsolavia)
Duillio Loi (Italy)
Myung-Woo Yuh, the 3rd most underrated i think (South Korea)
Rinty Monaghan (Scotland)

I expect your top 15 now with TRULY underrated champions,i hope i learn something from the answers :)