More than a few options here: 700c x 38, 650b x 42, 26" (559) x 1.75". And now 700x x 50.
The Marathon is Schwalbe's most famous, standard tire. It specializes in nothing, but is good for everything, and is super tough because it's stout.
If you truly don't want tire problems and you're sane or willing or realistic enough to know that with weight comes good things, but you don't want to go overboard with the SmartGuard liners Schwalbe puts in its ultrastoutest tires, then the Marathons are the tire to beat.
What Schwalbe calls 700x38 is actually closer to 36, the 700x50 is closer to 46, and the 26x1.75 is closer to 1.68. The actual measurements are based on 22mm wide rims so they'll be even less if you use 20mm or narrower rims.
It weighs 861g, it is German Stout, but every gram pays its way by providing longer wearing tread, tougher sidewalls, and extra puncture resistance. If and when you get past the weight demon and are just out for supreme reliability and great all-around rough stuff performance and all that, get this tire. Make sure your bike has room for it. Inflated (as you'd ride it), it stands 708mm tall. Compare that to what you're on now, and see if the clearance can swallow up a radius of 354mm.
To measure your current tire, set it up dead vertical and measure from the hub center to the floor.
You'll probably get 7,000 to 10,000 miles out of it. That's a prediction, not a guarantee, but it's conservative.
New for features for 2011:
-- Now with GreenGuard puncture protection, a 3mm thick layer made out of recycled latex materials.
-- Stronger, more durable sidewalls resist cracking at comfy low tire pressures.