Tom Sharkey wasn't the fourth best heavyweight in the history of the sport. Lewis arguably is. Sharkey wasn't as good as Dokes, Weaver, Bonecrusher or Greg Page. He was a pioneer. Different eras. Wildly different sizes. Wildly different skills sets. Ruddock was a very good boxer when he chose to be.
Tom Sharkey was a pioneer so that automatically makes him top 3 in skill set. Lennox Lewis wasn't a pioneer!! Therefore, Tom Sharkey stops Ruddock!