It's really hard to say, i'm not a tech guy, so i'll be talking out of my ass anyway, but on that side the one that impressed me the most were Agni's for cloth and hair stuff, Deep Down for the fluid simulation and general detail and of course Infiltrator, also for the detail and scope of the whole thing.
I would say Infiltrator probably the most impressive one here, but the fact that it's not aimed at consoles kinda factors into it not being as impressive as, say, Deep Down or Agni's.
Aesthetically speaking, though, i just love the photorealistic yet restrained style of Fox Engine/MGSV, particularly the Ground Zeroes part of it.
I love photorealism, but when it's really photorealism and not some hyper exaggerated cinematography à là Crysis 3 or Battlefield 4.
The sky and the colors in this look so beautiful.
Another example of what i'm talking about, would be ArmA 3, which can also get very beautiful in its realism, though not as much as MGSV.
I think photorealism can achieve a somewhat Malickian quality for me, and that's why it excites me.