This or the 24-70/2.8 TheEmrys mentions above. On my 6D I'd be taking the Canon equivalent 24-105/4L and a 50/1.4 as my fast prime; on my 60D (crop) I took a 15-85 and a 50/1.8 a few years back.
Given that the D810 is going to be fairly decent at higher ISOs (maybe better than my 6D, still waiting for comparisons
), a slower, but lighter and longer zoom like JAE mentions above will probably be the best bet for a balance of portability, flexibility, and image quality.
I'll also note that there's a new 28-300 coming from Tamron that has my attention; they are very small and light for what they deliver, and Tamron has really stepped up the game on their optics- but I don't expect it to do the 20MP sensor in my 6D justice, let alone the AA-less 36MP Exmor in the D810