If I had a brand new PC, I'd run... Well that depends what I'm using it for. I have a Surface Pro 4, which refuses to run anything other than Windows 10 owing to lack of driver support for GNU/Linux or BSD and I'm impressed with it, it's a good portable device.
My desktop dual boots Windows 10 and Ubuntu. I use both equally, I prefer Ubuntu for general browsing, listening to music and office work in Libreoffice. Windows 10 however for superior app support. Ubuntu is probably better for writing custom software.
I wouldn't recommend any version of Windows other than 10 for desktop use, simply because I'm not a believer in outdated software. 7 is now on security updates only and any older version is likely to offer poor security. I don't like the privacy aspect of 10, but feel if you're not too bothered about that it's a useful and innovative operating system.
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I think the concept there is missed slightly - RISC OS has no concept of *installing* anything - applications are folders - you can simply run them from wherever you wish, so locking down the ability to do so would be meaningless. What might be a better model would be blocking the *running* of apps that are not signed in some way.
It's slightly disc-space-inefficient because you end up with duplicate libraries, though. (There was the !System repository, but it wasn't heavily used). But did have the massive benefit that to "uninstall" something you simply delete it. In these days of huge amounts of storage revisiting this concept with some security added would probably be a good thing - then nothing could end up messing up your Windows installation.
This is a similar approach probably used by Ubuntu snaps, with the exception that you install them. They are however ran more or less sandboxed so could probably be adapted to work in this way.
Edit: RISC OS is still available by the way, free of charge for the Raspberry Pi. I would however struggle to recommend it for anything as it's fallen hopelessly far behind, no memory protection, cooperative multitasking, single user, poor internet resources. Nope, can't think of a singe practical use for it but have experience of using it at school back in the day. Even managed to rescue an A4000 and a RISCstation from my old primary, but those have long since been sold on eBay.
I think Ubuntu is a good OS, very rarely does one have to resort to the command line. Ubuntu MATE would be good for anyone wanting a traditional desktop experience and I recommend it over Linux Mint with its outdated kernel.