vmdk is the "raw" format
must be use in the case you need (like me)
live bkups

if you use Windows server the service VSS and vmdk as vm

format

permit to bkup VMs live when they are running

I don't trust backups that are made while something is using a file
(and the virtual disk is just one big file). To do it properly, the host
would need to, somehow, freeze access to the file, while it is
backed up, and that can take some time, depending on how big it is.