Conn's clearly among the best ten LHWs to ever hold that title. Making some distinctions here though, I think Charles and Tunney were greater LHWs, but of course neither ever held that particular world title. Certainly, as a divisional champion somebody like Moore would be far greater than Billy, who only posted three title defenses before going after Louis.
Ranking just titlists he has to be top ten. I'm positive, however, i'd rank him behind tunney, greb and charles as greats within the division.