On my formula hub, I have a 17t Surly cog

On my phil fixed/fixed I have a 15t dura ace and a 17t EAI.
I haven't had those last two cogs for too long, but I haven't heard anything bad about them.

As for my Surly cog. No complaints. I've had it on for over a year and there is no visible wear. I've worn chainrings down, but the cog has been in nice condition.

Don't go cheap on your back wheel.