I personally prefer 32-38 mm, 28 mm is my absolute minimum.

But remember, all tires are not created equal! Some people say their 32mm tires feel really sluggish, and they probably do, but not all 32mm tires will. I have some 38mm Vittoria Hyper Randonneurs on my main bike and they don't feel sluggish at all. Likewise the 32mm Panaracer Paselas on my Raleigh Gran Sport feel really great.