28mm-32mm but I haven't tried anything larger.

It was quite the revelation the first time I put on a set of 32mm and went out on my beat-up local roads, made the whole ride a more pleasant experience, no longer had to be hyper vigilante for road hazards. If I head out the other way, which has mostly decent undamaged pavement, the 28mm's do nicely. I don't have a bike with clearance for anything larger than 32mm, but I am curious.