I just had a conversation with a colleague about what might be called philosophy's biggest losers. We were trying to come up with a list of 20th Century philosophers who, although they were considered to be absolutely top-notch in their day, are at present completely forgotten and unread.
We initially thought of A. N. Whitehead. A towering figure in his day; no one cares about him now. But then we were reminded of the "process philosophy" hangers-on. So Whitehead isn't the biggest loser after all.
Norman Malcolm then came to mind. Seriously. Does any other 20th C philosopher match him? He was a major force in his day. But now nobody reads a word. Not one essay is considered essential. Not one essay is even regarded as historically significant in the development of subsequent philosophy. Malcolm is Philosophy's biggest loser.