Do you have a bike already? What size chainring do you have? Cogs are cheaper and easier to swap out and it's nice to have different sizes laying around in case conditions change. 48-18, 46-17, 46-16, 42-15 will give you gear inches in the mid-70's; a good place to be. If you've got mostly flat terrain, you might go to a higher ratio, 48-16 for example. Or if you've got a lot of hills and/or commute with a lot of stuff, go to 48-19.

Sheldon Brown has some useful info. on this subject and a gear calculator. www.sheldonbrown.com/gears Simply input your wheel and tire sizes, select gear inches from the pulldown menu, and input your chainring and cog sizes. You'll notice as you play with the calculator that a one tooth change on a cog gives a much bigger change to the gear inches than a one or two tooth change to a chainring.

Everybody rides a different ratio, don't feel that there's a "best" or "coolest" ratio. Although the ratio I use, 42-15, is pretty damn cool. Almost as cool as the ratio on my other fixie, 48-17. Now that is one bad-ass ratio.