I have a Surly cog which seems pretty good. Some of the cheaper cogs seems to be offset a little so the chainline isn't perfect. I also found that a prime number of teeth on the cog (i.e. 13, 17, 19, 23) will enable you to use any chainwheel size without constantly wearing on a single patch or set of patches on tire (this is the "skid spot" that you'll hear about here).
As for gear ratio, I currently run a 48x17, though a 46x17 (roughly like the 42x16 you currently have) was good too.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter