ben wallace averaged 12+ rebounds per game for 5 straight seasons, but I'm not sure that qualifies him as one of the two best of all time. 29.5 minutes per game and 9.6 rebounds for his career...
It's a mistake to use raw numbers for this discussion, again because of pace. Wallace, for example, posted those numbers during the slowest pace in the post-shot clock era, and his 12 RPG in the early '00s would have been closer to 15 RPG in the early '90s, or 16+ in the early '80s.
The best way to frame it is percentage of total rebounds available, a metric in which Rodman simply blows away the competition. He owns the top 7 seasons in league history, including one year in which he grabbed 40% of the rebounds available when he was on the floor. That's simply staggering, like hitting .400 in baseball - it doesn't seem possible.
To put it into perspective: Wilt holds the single-season record with 27.2 RPG in 1960/61. But he "only" grabbed 36% of the available rebounds that year. Rodman, at 42%, would have averaged 31.57 RPG.
It's kind of mind-blowing.
Wallace's numbers don't quite compare even by that metric, however it's worth bearing in mind that he was used first and foremost as a rim protector under Larry Brown, asked to chase shooters away and hedge hard on pick-and-rolls. Rebounding was a secondary concern, and obviously Wallace's late-career stats took a real nosedive as he lost his lift.
But when he was given license to own the boards, he was a monster.
I suspect that, in a modern scheme, Russell would've ended up being Wallace-esque on defense with a Marc Gasol-like offensive game. And Wilt would have been a lot like Dwight Howard on both ends.