Depends on what I was using it for. For general use I'd go Win8.1 and Classic Shell. And I'd upgrade to 10 when it was available...or maybe a few weeks after to see how badly MS is going to screw it up. I'm an Insider, so I've been testing 10 since the early builds, and I think MS is going to get this one mostly right, but still, waiting a few weeks won't hurt you.
For a gaming platform I'd stick with Win7. Games in 8.1 are faster, but I've had some compatibility issues and have gone back to 7 for my gaming. Also, if you're locked into legacy software or hardware that runs in 7, I'd make damn sure it will run in 8.1 and 10 before making the upgrade.
My desktop boots to 7, 8.1 and 10. I'm going to keep the 7 install for gaming and the 8.1 install because I'm an IT guy and I have to keep up to date. But I'm also advising my customers with 8.1 to upgrade to 10 a few weeks after release, or when I tell them to.