Personally, having used all 3, I prefer 7 as for me its the devil you know and technically it is a good platform, having said that, 8 isn't all that bad one you get the hang of it but it is really designed for a touch screen otherwise it can be a bit daunting.
With the lack of 7, Win10 is ok, and probably more suited if you avid 7 user, although for me, its still early days and a few more issues need to be sorted, be prepared to do tons of backups or disk cloning on 10, as at the moment when it goes wrong, it seems to go wrong in style.
My choice would be 7, but Ive got no cost access to it, if I had to go out and buy 7 I'd probably say forget trying to learn 8 and jump in the deep end with win10, after all it supposedly going to be with us for a while.