The only people who would know for sure would be IBM.
I would hazard a guess and say it's AIX -- because it was an AIX-only product for the first few years of its life.
While I personally recommend the combination of AIX/DB2/TSM/CMOD (I call it "One-Stop-Shopping for Blame when something goes wrong") there are lots of large systems (1TB+) on all the platforms. It's less about the platform, and more about managing it properly.
-JD.
