It depends on how far your trips are.

On my "fixed gear commuter," which often takes me over 30 miles in a day, I have a 44t ring with 17t and 18t cogs. 90 rpm speed = 18.5 or 17.5 mph.

My Miyata that I use for occasional commuting, but mostly errands around town has 42t ring with 15t and 16t cogs. 90 rpm speed = 19.8 or 18.6 mph.

My ghetto cruiser, which seldom makes trips over a mile, has a 42t ring and a 14t cog. 90 rpm speed = 21.6

As you can see, the basic premise here is that the longer the ride, the smaller the gear you want. My gearing choice for the commuter is based on the gear I'd use on my 27 speed road bike if it was flat and there was no wind and I was riding all day. It's geared just a little higher than that. I'd gear it lower if I was going to use the bike for a century.