|Radeon R9 295X2||GeForce GTX TITAN Z|
|Single precision performance (TFLOPS)||11.466||8.122|
|Double precision performance (TFLOPS)||1.433||2.66|
|Thermal Design Power (W)||500||375|
So the absolute winner is Radeon R9 295X2. Yes, it is a little bit power hungry, but its 22.9 GFLOPS/W is slightly better than the 21.7 GFLOPS/W of GeForce GTX TITAN Z. If you cannot afford to build a nuclear reactor in your room, the Radeon R9 290X is also a good choice with 18.78 GFLOPS/W.
Just a side note, but I really don't know what to think about GeForce GTX TITAN Z. Its FP64 performance is remarkable, but the board is a bit overpriced. It is also an open question, how the board performs using OpenCL, since NVIDIA's support for CUDA is far better than for OpenCL.