I'm currently on Win7 but I am making preparations to move to Win 10 by the end of the year to retain Steam compatibility.
As far as what OS you should use for older games depends on how you want to be running them. If you want to be using real hardware/software or if you are fine with using emulation. If using emulation then it doesn't really matter except that you'll need to use (at least) a professional SKU and use a CPU with a vt instruction so you can run a hypervisor. Else you can go towards a Linux solution and use something like ESXi to be the hypervisor. No matter what HV method you choose, you are going to run into compatibility issues, so you'll need to research the different options like VirtualBox, Hyper-V, VMWare Player or ESXi or another solution.
If you are going to hardware level, then it comes down to what games you have already or what you want to play.
I already have a Win98 PC, which I've had for over 20 years now. I also plan to make a Win95, an XP and a Japanese Vista system. Of course I have the hardware and software resources to build many different PCs that many people don't, so I can't necessarily make those types of recommendations. For example, I have some games that require specific hardware, to generate effects that cannot be emulated. Those are going to be 90s games that require specific video cards like a 3DFX card or ATI Rage.
As far as the security certificate situation, I did not know that was involved with reading CD/DVD based games. I know that issue exists for networking hardware, which is why I have to keep 2 notebooks in my notebook bag (makes it heavy) where one has XP to use internal pages on old hardware.
