Krytac hands down. They have the most, best parts in their platforms, even in their Alpha line, they shoot phenomenally.
While G&Ps are very solid performers, the only real things they have change up is their externals. Though one thing G&P has been doing right for some time, is coming standard with Deans connectors.
Ares have been changing and trying new and different things with their systems and guns, but from the things I've worked on, they are still not all that fantastic. Most of their mosfet systems that are programmable, require a separately sold programming unit. And the mosfets themselves aren't exactly the most robust. More robust than the G&G ETU from what I've seen though. And Ares still has their weak points I'm their mechanical system such as their bushings and the over all proprietary design of the shell and mosfet.
VFC is indeed stepping up their game as of late. They have mosfets in the same general location as in the Krytac, though with their standard shell with a little modification. A basic mosfet, which I haven't had too much experience with their longevity, but I imagine they should be just fine. They also tried to improve their piston and tappet plate material. While better than before, they are both still the weak points in the system. But overall a great platform, VFC is known for phenomenal externals and solid performance.
As for APS, they have had rough beginnings and have improved, but I cannot say too much as I haven't had much experience with their more recent stuff. Some of which, I must agree, looks really cool. But on an internal and performance level, I don't know.