My suggestion would be that you hold off on a wide prime if you're buying it for the future. Any of them are okay if you're looking to match them with the D700's sensor, but the D800 (and, one would assume, its successor models) has about the same pixel density on the sensor as the D7000, and the Nikkor full-frame wide angles are showing their age on that model (and not well, either). They're not particularly sharp at the corners, and display significant vignetting already -- and that doesn't account for performance at the edges of a full-frame sensor. If you can, try to get some feedback from people who've tried them on a D3X, but I wouldn't expect any rave reviews.

Since Nikon has decided to step into the miniature medium format world -- using a sensor that's verging on the very edge of even theoretically perfect lens performance -- and their stable of full-frame wide-angle primes is getting a little long in the tooth, I'd expect to see some newer design trickling in over the next few years. So buy for the short term rather than for the future if you're waiting for a successor to a model that isn't actually shipping yet -- something that might show its age on a D800 could be the best thing that ever happened to your D700. And get the focal length you need rather than worrying about which one is "best" -- the best lens is the one that lets you take the pictures you want to take, not the one with the best specs, build quality or handling.