[NullS3c]

macOS Sierra or Linux Ubuntu(personal use)/Kali(pen testing obviously).

Debian to: Server, MySQL/MariaDB, MongoDB, PostgreSQL, C++, Assembly x86_x64, Python 3
Ubuntu/Mint to: Node JS and Python 3
Kali Linux (+ whonix gateway) to: Pentest and Computer Foresics
Windows 10 to: Game, Art & Design, C#, Ionic 3, Angular 4, Computer Foresics, Reverse Engineering and other things

A diverse setup dedicated according to usability. I don't come across too many users who segregate their computing to this degree.

Are they running as physical hosts, or are some as guests (VMs)?

In my notebook i use ubuntu/mint, and in my desktop i use Windows 10 as host and 16 VMs (Linux). To run multiple VMs i have 64GB of RAM.