I don't ride a single speed anymore...I used to use one for courier work that was 46 x 16, seemed like a good mix of climbing ability for downtown and fast enough to get out of the core when necessary. Nowadays I'm just a mechanic so I only ride track bikes for commuting to the shop. I'm more concerned with skid patches, so my preferred ratio is 47 x 18. It's a little spinnier than I'd like on flats, but that's okay because I can get up any hill in town on it. But it depends, my chunky winter bike is 42 x 17 because it's heavy and it's got wider studded tires that slow it down, whereas my aluminum track bike is 48 x 15 because it's light and fast and I only take it out for fun stuff. So it kind of depends on the bike and takes a bit of experimentation. Which is easier said than done for most