Okay, this is assuming a lot, but assuming that I'm limited to picking ONLY ONE device, and it has to be one of these three, and I can't get another one later, then this is a VERY EASY question for me:
The Surface, no doubt.
The other two are tablets with a keyboard. ONLY the Surface is a TRUE 2-in-1, providing ALL the functionality of a laptop, -AND- ALL the functionality of a tablet, and switches between them with the snap of a keyboard! The other two are ONLY tablets, and as impressed as I am with the hardware on each, as well as being quite pleased with iOS and especially Android, this is still a "no-contest" situation for me. The Surface.
Even the app gap doesn't dissuade me, because a) all signs point to it improving radically in not too much time, and b) even if it doesn't, most of those apps have win32 or web-based equivalents that I can use when in desktop mode and/or via Edge.
However, my REAL answer, no longer playing along with the parameters of the thread is "I want an iPad, AND a Pixel, AND a Surface." And assuming I would get all three, then it only comes down to the matter of "firing order", which once again, is a very easy question for me.
I have an early 2015 iPad Air 2 already WITH an aftermarket bluetooth keyboard cover - in fact, it's what I'm using to type this. So, the iPad pro would definitely come last. No question there. Next, while it's admitedly only a very pale shadow of a real Microsoft Surface, and indeed, was only ever intended to serve as a stopgap for me, I already have a Nextbook Flexx 11, which more or less approximates a Surface, and is serving me just fine as an intermediate tide-me-over. However, what I DON'T have is any meaningful solutions for an Android tablet. I have a 2011 HP Touchpad, which originally ran WebOS, but has been rooted, and hacked to run cyanogenmod12 (Android Lollipop.) It's technically usable, which does make it better than nothing, I suppose. But it's soooooooooo slow, and sooooooooo buggy, and often crashes, that it is not a solution at all for anything more than a rare super casual dabble with Android.
So, without a doubt, the Pixel would be the one I'd get first chronologically, followed by the Surface, and again, the iPad Pro would bring up the rear. But once again, assuming I could ONLY have one - EVER, then it'd definitely be the Surface.
Cheers!