most Mac-compatible linux distribution I want to install Ubuntu on my iMac. But I don't want to use Unity. I want to use Gnome 3. I can install it over Ubuntu (as I do on my laptop) but I know that Fedora and OpenSuse use Gnome 3 natively.

Also, I use Ubuntu on my laptop because of drivers. It was a pain in my ass to install all working properly on Fedora and CentOS. So finally I've installed Ubuntu and it works fine for years.

So now I understand, that here is another bunch of drivers on my iMac and maybe here won't be any problems with Fedora.

Ok. Actually, the question is what is the best distribution (best, I mean most compatible with drivers wireless devices: Mouse, Keyboard etc) to use on iMac. As a second OS and as OS in my VirtualBox.
I look at this issue the other way (which may not help you at all as you mention video driver issues and you want full linux access to the GPU).

My take is "Which virtualization environment works best with the linux distributions I want to use?"

For me the answer is VMware Fusion, then VirtualBox and lastly Parallels. And I really haven't heard any significant issues with all three of these in the last several months.

The reason is that my time is so much more precious than whatever marginal increase in speed I would have gotten installing linux natively. If and when I need huge performance I deploy to co-located linux service where you can easily pick your optimal price / performance / service level.