Agreed with comments of James Bradbury.  If you are building/buying your own system look at the Titan. If you using a cloud-pricer, AWS uses K40, although you can use multiple GPU depending on the P2 instance size you are using. It is my understanding that AWS will be supporting the NVIDIA V100 this Fall.  The performance should be at least as good as the Titan X, if not better (https://devblogs.nvidia.com/parallelforall/inside-volta/ 13). Disclosure - I work for AWS.
Nick