A defensible stack rank, in my opinion:
1. Kareem - inarguably a top tier scorer, excellent defender and rebounder, very good passer, solid FT shooter, clutch during endgames, tremendous longevity, won at every level. Faced Wilt, Thurmond, Walton, Sampson + Olajuwon, and won Finals MVP by owning Boston's vaunted frontcourt in the 1985 championship series at the age of (I think) 38. Flawless resume.
2. Shaq - excellent footwork and surprisingly mobile given his mass, good defender when fully engaged, capable passer, dominated multiple Finals series. Awful FT shooter. Played against a fairly robust set of opposing centers. Could've had an even better resume if he'd kept himself in shape.
3. Wilt - tremendous scorer but offensive repertoire was limited (footwork and pivot play not nearly as sophisticated as Shaq, for example), very good passer, excellent defender. Awful FT shooter (in the 38% range some seasons, and playoffs). Numbers inflated by insane mpg (averaged 48.5 one year), the league's general FG% incompetence in the 60s, and high pace of game: far more rebounds to go around, and so on. Failed to dominate Finals, and playoff scoring stats not nearly as gaudy as regular season. His Finals scoring numbers are particularly puzzling at roughly 19ppg and 38% from the line. Limited to 11.7 and 11.6ppg respectively in the 1969 and 1973 Finals (both losses). Faced robust competition at center (Reed, Thurmond, Lucas, etc.) but many would be considered undersized compared to today;s game. All this said, Wilt was likely the greatest athlete out of all the top centers (excelled in track, volleyball, etc.) - but the game simply wasn't as sophisticated back then, and skill sets fall far short of modern players.
4. Olajuwon - incredible footwork and agility, dazzling post moves, very polished offensive game, arguably the greatest shot blocker if one adjusts for evolution of the league from the 60s to the 90s, solid FT shooter. Excelled in Finals series. Faced and fared very well against Shaq; pretty much owned Ewing.
5. Russell - the consummate teammate and winner; perhaps the greatest defender of all time. Very limited offensively. Terrific rebounder, very good passer. Awful FT shooter (but still better than Wilt). Playoff stats better than regular season. Excelled in Finals series. Stats inflated due to mpg (mid-40's most seasons), pace of game... all the same factors the affect Chamberlain's numbers. Given the evolution of the game, one might consider ranking Russell on the list of PFs, either #1 or #2 behind Duncan. Won at every level. Faced and fared well against Wilt; remainder of competition at center solid but undersized by today's standards. Skill level just not as sophisticated / polished. Similar to Wilt, tremendous athlete. Would've been a terror at PF today.