I use 49x16...that'll get me down the open road pretty fast with moderate hills..any higher and I'm slow going up or against the wind...any lower and I'm spinning out on the downs and in the flats. However, it took me some time to get strong enough to push that gear fast mile after mile. In the city, I'd go with something a bit lower. On the track I'd go a little higher.
My beater has 45x16 and my ssmtb, which I use in the snow, is 34x15, 16, 16, 18 (it varies).

Whatever your gearing...you still need to be able to really spin at a high cadence if you want to go fast with a 1-geared bike.