The best rig possible would have an i7-5960X processor, Haswell-E motherboard, 32GB or 64GB Corsair Dominator Platinum DDR4 RAM, two GeForce GTX Titan X's in SLI, a 1TB Samsung 850 Pro SSD, one or more 4K 27+ inch monitors, audiophile audio equipment etc. It would cost about as much as a new Polo.
I would say quad sli titan x on a 5k led display. reality of the situation is once done it would be the cost of a flat somewhere on the coast
It would be a waste though as a few months later there would be something much better out there and you would have some serious buyers remorse.
Me I would go for a 2560x1440 display which a single Titan would handle very well. I've been playing a lot of FC4 and truth be told when things are moving around you don't really notice the difference between ultra & medium running through the trees. If you stand still and pan slowly everything maxed out looks awesome.
Gforce experience is also a crock of schite as they always go for lower settings when you can do much better going by tweak guides for your gpu.