Lennox is an easy pick for top 5-10 of all time. You gotta remember he demolished Razor Rudduck, who gave a prime Tyson fits. Lewis has dodged no fighter out there and that is one of the main reasons I give him respect. He has made fighters like Michael GRant and Andrew Golota like worse than sparring partners, Easily beat Holyfield in the first fight and did enough to get by in the rematch. Made Tua look like a joke, avenged his only two loses in grand fashion, and dominated an old Tyson. This guy is an all time great. I agree the era of the late 60's and 70's had many more tough Heavyweights out there, and that is one of the reasons you will get a lot of people saying that Lewis would not have been as high on the list if he would have fought in that era. I still think his size and skill would be enough to overcome most from that era.
Marciano was mentioned, and yes his record was good, but it was mostly just a record. He fought Joe Louis after Louis was already retired and was making a comeback because he was broke. Jersey Joe Walcott was past his prime when Marciano fought him, and Marciano fought his own brother twice under an assumed name to add to his record.
Other Heavy's that could be top 10 material.
Sonny Liston, Jack Dempsey, Ezzard Charles, Joe Frazier, Ken Norton, Gene Tunney, and Floyd Patterson.