173... mostly because that's what I've gotten used to. I tried putting with a 168, and it just "felt" wrong to me... and I didn't realize the weight until I looked on the inside. I doubt 1 gram or so one way or the other would make a difference to me, but a few certainly did.
Also, while I often prefer lighter weights (in the 160's and down to 150 and even 139 for Blizz) for some drivers, I like the idea of using heavier putters. All other factors being equal, a given wind should have less effect on an object of greater mass (at least for the vast majority of circumstances).
As for driving, I can see using lighter putters, but I don't think I'd like that. Rather than carry another putter just for driving, I'd prolly stick with my Challlenger that "feels right" in my hand or go to a mid for that shot.