We're finally at the big one. I think the contenders for the top spots are (ordered by era):
Bill Russell: Probably the defensive IQ goat. The best shotblocker after Wilt and Wilt chased blocks at times. Best rebounder after Wilt. He got to rest on offense more than most of the centers on this list though as he was mostly a defensive specialist and was a system player on offense mostly. He would routinely mind game players into taking bad shots. He not only blocked a ton of shots, he was a master of tapping his blocks to teammates for easy buckets.
Unofficial block data for Russ and Wilt:
https://forums.realgm.com/boards/viewtopic.php?t=1413138 (Russ 8.1 over 135 games, Wilt 8.8 over 112 though 30% higher pace than modern times)
Wilt Chamberlain: Best rebounder and shotblocker of all time. If he was locked in, he was absolutely the most dominating defender. Did tend to chase blocks too much, and would not necessarily try to utilize his strength as much as he could or perhaps should have unless someone pissed him off. He almost retired early in his career at points due to how much abuse he took in the paint, similar to how people would literally hang on Shaq and the refs would give players a lot of leeway because he was so much stronger than his peers. Was a smart player, but not as smart defensively as a some others on this list.
Nate Thurmond: Among the top 10 rebounders and shotblockers in history and could hold his own against Russ and Wilt, though somewhat overshadowed by them. Was built like a tank and pretty quick. Reminds me of an old school David Robinson.
Kareem Abdul-Jabbar: Not only a top ~5 peak rim protection, he has the career impact as well with his nearly 20 years of star level defense.
Hakeem Olajuwon: Top 5 rim protector and probably 3rd best after Russ and Wilt. Was a better man defender and ball thief than most centers as well..perhaps the best in the group. Though Russ still almost certainly has the IQ edge Dream has a very high bball IQ.
Dikembe Mutombo: Defensive specialist so he got to rest a lot on offense, but earns his spot with a top ~10 rim protection case and probably closer to 5, along with durability matched only by Kareem in this group. Not quite top 3 man defender, but he was a nightmare matchup for any opposing center as he was relentless.
David Robinson: Maybe the best all around athlete of the group outside of Wilt and David is quicker. Very smart and unbelievably good all around defender. Was amazing even in his rookie year. Unfortunately not a very long career. Top 5 peak rim protector though in my book and top 3 man defender/ball thief in this group as well.
Ben Wallace: Top 10 rebounder and shotblocker and top 3 man defender and ball thief in the group.
As for my vote...well I think it is closer than some are stating. But ultimately I think the top spot is between Russ and Dream for me, with 2 or 3 others on their heels. I'm going
Bill Russell.