Whatever allows me not to go below 1/125th. I do mostly street photography so I use Auto Iso up to 640. As I like to shoot wide open, most of my day pictures are at 160 which I find wonderful with the M8.
I don't like the "1250" and find the 1600 R-D1 grain way better, similar to the 640 M8 grain.
2500 is unusable IMO unless you have to, that is below 1/8th.