For a single year, Mourning might have done the best defensive job for a center in the era in that 1999-2000 season, which for me as an owner of Shaq, kinda sucks because it makes people forget what an unbelievably great job Shaq did that year as a defensive center. Many other years, the year Shaq put in on defense that year may have won him a Defensive Player of the Year award. Instead, because of Mourning's unreal dominance as a defensive player that year, he only got 2nd team All-Defense. Most people forget Shaq was runner up for DPOY that year and hence people forget what a beast he was defensively for that season.
That year and the year before Mourning got all sorts of accolades for his defense but he could never really and even in 2000 could slow down Shaq. Shaq scored over 30 PPG and shot over 56% for his career against Mourning and even in 1999-2000, Mourning was only able to slow Shaw down to 28 PPG and 54% FG% in three games.
No, Mutonbo was the best that I remember versus Shaq.