Very, very good but could have proven much more as champion. I am happily enjoying @apollack 's second installment on Dempsey right now. The estimation of his contemporaries was through the roof but there were similar voices touting Wills. Despite his shortcomings as champ, he still easily gets in the top 20, even top 15.
He was a great h2h fighter for his time, but terribly overrated in terms of all time greatness. Very weak resume and a dreadful run as champion. People criticise fighters of today for being inactive but Dempsey as Champion was worse. 5 title defenses in 7 years is ridiculous. I usually have him in the 20-25 range all time
Great fighter but through away his years as Champion. Dempsey's style required frequent bouts or he was going to go stale and he did. Too bad he just wasn't more active after he won the belt. Probably a top 12 for me
He was one of the best finishers from any era, and perhaps the best heavyweight of all time, at overcoming a size disparity. As others have said, his title reign started with a bang, and ended in disappointment.