I use Continental Ultra Sports 27 x 1 1/4. I think the Gatorskins are probably better tires, but they cost twice as much. I don't get alot of flats, so the extra money for the gatorskins didn't seem worth it.

Before buying anything skinnier than 27 x 1 1/4, look at the insides of your rims. Some older 27 inch rims don't have ridges to hold the wire bead of modern tires. Such rims won't support more than 80 or 85 pounds of pressure. So, in that case, you are better off with the wider tires that don't require the higher pressure.

Edit: Personally, for commuting, I like the wider tires anyway. Skinnier than 1 1/4 is a racing tire as far as I'm concerned.