Which system should I choose when installing OPNsense on Proxmox VE?

Hi,

what do you plan to install exactly in that VM? A nested instance of Proxmox VE, guessing from the rest?
If so, you'd choose Linux.

And for further details on that topic, please see the Nested Virtualization wiki page in our documentation.

Hope that helps!
 
I came here by search looking for something else, but since I'm here I'll leave a note for those wondering:

The reason that it asks about OS is that it will use that info to preselect other choices.

For example:
Windows 10 and 11 need a q35 with a VirtIO installer ISO, as well as UEFI and TPM
Linux supports VirtIO natively, so everything VirtIO will be selected by default
Other (unknown) will pick i440fx with an IDE disk and E1000 as the network card - because they're the most widely supported, and targeted even by the academic OSes like Plan9

In the case of FreeBSD (OPNSense), much of the support will be similar to Linux - consider how TrueNAS (also FreeBSD) is used as a hypervisor, somewhat similar to Proxmox. OpenBSD or NetBSD