Formerly Known as Newbie
Join Date: Mar 2002
Location: Helsinki, Finland
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Cheers jedwards, welcome.
A couple of things to consider. Start by looking up the range of widths you can safely fit on your existing rim - for this, see Sheldon Brown's tyre sizing page
(roll down to the table with red/green squares).
After that, it's basically a question of what kind of conditions you'll be commuting in. Do you need puncture protection? Schwalbe Marathon Plus is one of the absolute best in that regard, but it's also very heavy. If you're riding on paths or other loose surfaces, you might not want to get the narrowest possible tyre. Etc.
Finally, remember a bike cannot hydroplane. On pavement, tyre surface pattern ("water-shedding" thread vs. slick) will not have any effect on grip.