I'm riding Marathon Supreme 700x35. But I understand that's a big bill and won't suit everyone. (Perfect as far as I'm concerned.)

I've also tried:
Marathon Racers (the same nice grip as the Supreme but not quite as good flat protection)
Hard Case Lite (I like the one I've got but it's a 23 so it stays on spare duty)
Transworld City (I can not recommend, too many flats per mile for my taste)
Armadillos (Love the flat protection, hate the ride, especially on wet pavement)
Kojaks (Great ride, I don't think they're supposed to be flat resistant, but my wife hasn't had a flat in 2000 miles)

The reviews here for the Gatorskins are very good. I would strongly consider them too.