I can now say with some confidence that the best currently available GPU for mining Grin is the NVIDIA RTX 2080 Ti, with one caveat.
The main reason to prefer the 2080 Ti over the 2080 (and 1080) is the 11 GB memory which allows it to efficiently solve the ASIC targeted PoW, cuckAToo31+.
The reason to prefer the 2080 Ti over the 1080 Ti is the much higher efficiency at running cuckatoo31. Even without using its claimed 64 KB shared memory capability, it’s over twice as fast, and I don’t think its power use is close to twice as high.
The caveat I mention is that when ASICs show up, they may have enough efficiency advantage over the 2080 Ti to relegate it over to the GPU focussed, ASIC resistant PoW, cuckARoo29, where it has to compete with many other cards that only need 6 GB.
In the rare event that ASICs don’t show up in the first year, and cuckatoo31 gets phased out, then the RTX Titan 24 GB looks well positioned to solve cuckatoo32+ efficiently. But at $2499 it’s over twice as expensive as a 2080 Ti…