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ESXi loads its OS into memory during boot and then runs from memory. From time to time it writes configuration data and (depending on the configuration) log data.
So ESXi OS itself doesn't have special RAID requirements - well you probably want some redundancy so don't use RAID0.
I'd say RAID configuration should represent the requirements of your VMs and their workloads.
Example: I had a case where a VM would have a very high sequential write rate. RAID 6 might provide good redundancy and capacity here but terrible I/O performance.