What type of 700c tire is best at guarding against flats?
Many brands (Michelin, Continental, etc) offer tires with puncture resistant casings. I have had very good results from Bontragers line of 'Hard Case' tires. Of course, you'll only find them at a Trek dealer. :-(
Wheelies don't pop themselves. (from a QBP fortune cookie)
Some of the Kenda tires are puncture resistant as well. My road bike has 700cx25 Kenda Kriterium tires on it and they are advertised as puncture resistant and run around $35.00 USD each.
HCFR Cycling Team
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Specialized Armadillos, Schwalbe Marathon Plus, and Continental Gatorskins are all very popular, durable tires with good reputation.