The base OS is going to be Windows, so right off the bat I'm going to want a Linux instance. There's just too many handy tools that are Linux-based, particularly when it comes to networking. You can also download purpose built OVA files (prebuild operating systems) for penetration testing, and what not. One such image is called Cisco VIRL, which can simulate an entire network on a PC. A decent VIRL setup needs about 32GB or RAM and a few cores by itself. On top of that, I have transient requirements to evaluate various network appliances such as firewalls, which many can be loaded virtually into VMWare.
It's also handy if you want to go to a questionable website that may be infected, or you even want to look at the effect of a virus. I can spin up an encrypted VM, and totally sandbox what I'm doing.
That sounds interesting. Way out of my level of knowledge, but interesting nonetheless .