Single Speed Cogs

So I've gotten a few rides on my newly assembled single speed 'cross bike.  One thing I didn't give much thought to was cogs.  The following doesn't really apply to you unless you're using a normal geared wheel with a single cog and spacers - a common setup when converting a geared bike to single speed.

See, you can use a cog from an old casssette you've got lying around, like this:

But it's not ideal because the teeth aren't very beefy, and some of the teeth are machined (ramped) to assist in smooth shifting, guiding the chain OFF the cog.  The unfortunate result is your chain MAY either skip, or come off. 

If you are using one of these cogs, it will really help if your chainline is really straight.  If that's not possible, it will probably be fine if your cog is a little too close to the wheel (i.e. towards the inside), as the ramps are on the outside surface.

PROS: Cheap, readily available in the pack-rat's box of parts
CONS: Possible chain drop/skipping from shift ramps, not as durable

You can also use a BMX/Single Speed cog:

The teeth are longer, which I guess helps the chain get a better bite when catching it.  There aren't any ramps to guide the chain off the cog, either.  They seem a little more beefy, too, so they'd probably last longer.

The nice single speed specific cogs have a wider base, to help keep the cog from carving up the freehub body. And they are fairly expensive ($25-$30).

PROS: More reliable drivetrain, more durable, wider base nicer on freehub body
CONS: Kinda expensive...$30 for a cog?

Shimano makes BMX specific cogs, and they sell for about $6 each.  They don't have a wider base, but they have the beefy teeth designed for single speed use.  Unfortunately, they only seem to go up to 18 teeth in size, which might not be big enough for you if you're using a 130 BCD crankset like me (common road crankset size).  Using a crankset with a smaller bolt pattern (e.g. 110 "compact" cranks) would allow you to use smaller chainrings, such that 18 teeth would be as big as you'd ever need.

PROS: More reliable drivetrain, more durable
CONS: bigger sizes do not exist (sizes range from 11t to 18t), skinny base not freehub body friendly.