As you've correctly identified, CUDA is the only sane default choice in the GPGPU world. OpenCL 1.2 is a viable distant second if you're sure you don't need any of the major CUDA libraries and value the portability aspect (not the same thing as performance portability, of course, but still). Finally, if you don't need strict accuracy guarantees and your application is mostly about graphics, you can get by with compute shaders. Everything else - POCL, HCC, ROCm, the various SYCL implementations, etc. you can safely ignore if you want something usable today, none of them are production ready or anywhere close to it.