The most forgotten is so forgotten that its likely he won't be mentioned here. Jack Root came to my mind when i first read the title, surely not the most forgotten but mentionable.
Gus Lesnevich and for good reason, he held the Light Heavyweight Title hostage..refusing to defend against Bivins, Maxim, Charles, Marshall, and Moore. The Adonis Stevenson of his time.
The answer to this question has allot to do with the fighters original popularity and celebrity status when in their prime. This being the case one has to look at the hwts as this has always been the most popular division among the vast populous. Marvin Hart was not very popular as champion so 110 years later of course he has been forgotten. My choice would be Corbett, Fitzsimmons, Jeffries and Johnson. All worldwide known and immensely popular in their prime. To the general population completely unknown today.