Pound for pound doesn't really enter into it with heavyweights! From around 185 lbs. on up, it's anyone's game. Beyond a certain limit, size and weight don't add any advantage. So the list remains the usual suspects: Sullivan, Corbett, Johnson, Dempsey, Tunney, Louis, Walcott, Charles, Marciano, Ali and many names between and since.
From Memory the last HW to hit #1 on the P4P lists was Mike Tyson. He did so 3 consecutive years 87-89. Evander Holyfield was as high as number 2 I believe but after that most HW Champions since then really only make the bottom half of the top 10 list if at all.
Tyson was the only one ever at #1 I think. Holmes usually hung around between 8th and 12th when KO mag first introduced its pound-for-pound rankings.
Just looking through a few of my mags from recent years, and in regards to Lennox Lewis, the highest placement that I've found for him was 9th, which was his postion in the p4p rankings for the May & June 2003 issues of The Ring. The May 2003 issue features the year-end "best fighter" poll as elected by the "experts", though, if you wanted that, which would show this; -Larry Holmes' highest finishing position was 3rd in both 1983 and 1984 -Pinklon Thomas finished tied for 8th in 1984 -Michael Spinks finished 3rd in 1985 -As stated by others, Mike Tyson finished first in 1988 and 1989 -Buster Douglas was 5th in 1990 -Evander Holyfield's highest placement was 3rd in 1991 -Riddick Bowe finished 5th in 1992 -Lennox Lewis' highest placement was 7th in 2002
Well I think Louis was ranked number 1 pound for pound any were from 1938-say 1941 or so. Pound for pound, Fitz has that title.