I like my Panaracer UrbanMax 32 mm tires ($15/each from Performance). I have been using them for about 4 months now without a single flat, even while my commute has doubled in length recently.

They are quite thick and flat-proof (the reason I got them!!!) but in my experience the rolling resistance compares favorably to a thin 28mm slick such as the IRO Duros (my previous tires). In the photo on PB's web site, they appear "knobby", but rest assured that these are just cosmetic grooves, and they run very quiet and I don't believe these affect RR.