The Best GPU render?

Quick comparison of GPU renderers

According this NVIDIA list you can find the best CUDA based GPU solution for your business. 
V-Ray RT is more like previz render for CPU version of V-Ray. It doesn't support all features like regular V-RAY CPU.
Octane in version 1.0 is quite fast, but doesn't support displacement, hairs and the total number of textures is VERY LIMITED.
Also Arion render has many limits and it's more like CPU+GPU hybrid.
We have to say, that the most promising and the fastest renderer looks FurryBall RT in our opinion. Only this render supports displacement, hairs, Maya fluids and particles and almost all features like regular CPU renderer. You will got for free all three plugins (Maya, 3DS Max and Cinema 4D), so it's not such expensive for 499,- 299,- EUR  ;-) There is also a possibility of the monthly rental for fair price from 49,- EUR. They also provide Credit version for free without ANY watermark also for COMMERCIAL use.
We are preparing deeper review of FurryBall and other renderers - please stay tuned.
Update 4/8/2017 - We prepared for you the price compare of GPU and CPU renderers.
Optix   NVIDIA OptiX Ray Tracing Engine™
OptiX is a programmable ray tracing framework for software developers to rapidly build ray tracing applications that yield extremely fast results across NVIDIA GPUs with conventional C programming. Unlike a renderer with a prescribed look, or being constrained to certain data structures or a language limited to rendering, the OptiX engine is extremely general - enabling software developers to quickly accelerate whatever ray tracing task they wish and execute it on widely available hardware - all license free.
Arion   Arion is Fry Frender’s next generation light simulation product which uses NVIDIA® CUDA™ to accelerate rendering on single or multi GPU systems. Arion is physically-based light simulator developed by RandomControl making it a high performance production renderer that can use all GPUs and CPUS in the system simultaneously.
FurryBall   FurryBall is a unique, real-time, GPU production-quality, final-frame renderer with advanced rendering techniques. It can be implemented directly into Maya and 3ds Max with network rendering and multi-GPU support. FurryBall 4 features NVIDIA OptiX™ physically based, full global illumination, and supports biased techniques. It combines the speed of GPU rendering with the quality and features of CPU rendering to deliver proven results in real movie and game production by multiple studios.
Learn more about FurryBall.
Lightworks   Lightworks has taken advantage of NVIDIA OptiX to create a new breed of ultra fast GPU accelerated ray tracing engines for the Architectural, Industrial and Interior Design industries.
Octane   Octane Render is an interactive GPU based, un-biased, physically based renderer that produces photorealistic results fast. This allows the user to create stunning works in a fraction of the time of traditional CPU based renderers. Refractive Software who develops the Octane renderer was one of the first to release a photorealistic and spectral based rendering engine which has significantly contributed to the Rendering Revolution. 
V-ray   V-Ray RT GPU is the GPU rendering mode enabling V-Ray RT to accelerate ray tracing on the available GPUs installed in the system, rather than the CPU. To enable GPU rendering, select OpenCL for the engine type parameter in the V-Ray RT settings. The V-Ray RT GPU mode is available for Autodesk 3ds Max.