In no special order...
1) Marciano: condition and punching power count a lot in a division where the skill level isn't generally as high as in some others. Also, he is pretty underrated defensively, and for his ability to adapt and adjust during fights.
2) Dempsey: very fierce, fought at a quick pace for HWs. Ray Arcel called him the best.
3) Louis: most effective puncher ever, very fast, kill you with any punch. Eddie Futch called him the best.
4) Johnson: huge historical impact. I don't know enough about him to assess his skills, except that he lost to Willard in a fight I think was probably legitimate.
5) Jefferies: very athletic HW and huge for his day. Very widely respected for many years after his reign. (Nat Fleischer rated him very highly, if I recall) People tend to forget that he fought Johnson after a 5 year layoff.
6) Tunney: I guess everybody knows by now that I am a huge Tunney fan, possibly the role model in my life. I always think that a thoughtful guy that can move, box and punch has an edge.
7) Tyson: at his pre-Douglas peak he was really something. Largely, I think, because he fought like a featherweight. His peak was short lived because he stuck with the peek-a-boo style, and as soon as his reflexes slipped a bit he became a target.
Holmes: real good fighter. moved well, sharp straight right, effective right uppercut, great jab. Got hit with right hands and fell down but got up. A better version of Ali.
9) Lewis: pretty good fighter that got to be a very good fighter once somebody taught him to use his attributes and fight smart. But his last fight, man, that was sad. Terrible skill level for a HW world championship fight.
10) Charles: 8 defenses, wasn't it? Great, great fighter. Given his success at MW and LHW, maybe the best of them all. I always try to give him his due. And I think he'd have beaten Frazier (watch his first with Marciano), Patterson, and the rest of that group of "great" heavies. Unless Foreman caught him, or if he couldn't handle Liston's jab.