I only just installed Win 8/IE10 in my work area, so I haven't had time to really form an opinion on it.
My current favorite is my Win 7 laptop. This is the machine I live out of, so I have it fine tuned to suite my tastes perfectly. I wish it had IE9 on it, but I haven't made the switch yet. I try to keep my main machine as representative of the current state of the web as I can. Though IE9 is gaining fast, IE8 is still ahead in the stats that I regularly read.
I do have a soft spot for my XP machine. I have had it for 7(?) years now and I have a lot of miles on it. It is underpowered, has a small disc and not much memory, but I still like it. I find it to be a good representation of on older, lower mid-range machine that someone might have running. You can't always assume that a user will have a lot of power at their disposal. If something runs on my old XP machine, it is fine. Sort of a sanity check.
As for VMs. Yeah, some of these could be combined together, but I see no need to. I like to have a separate monitor for each OS/browser combo, so no savings there. (I only recently moved to the KVM with Vista and XP to make room for Win 8. It was quite an internal struggle to put them both on the same monitor). Also, this way it is very easy to power up whatever machine I need at the moment and power it back down when it is not in use. That can become more complicated with VMs. Right now, the only machines that are powered on are my laptop and the Mac. (I play music from the Mac, so it is really functioning as a Stereo right now).