+1 on the Vittoria Rando Pros in 1.5 inch. I've done a little bit of loaded offroad on these and they perform well. Great all around tires, I use them in-city on one of my bikes too. Recently Nashbar had them on sale for about a third off what they normally go for. Knowing Nashbar, they'll likely go on sale again in the not too distant future.

I've been trying out a plain Schwalbe Marathon on my commuting rig for nearly a year, and I have to say I've been impressed. Other than a single puncture right after I got it, it has been bombproof despite my being a Clyde (~275) and usually carrying my load on the rear wheel where I have it mounted. It doesn't show any sign of wear so far, after nearly 2,000 miles. I originally thought I'd need a Dureme or one of the other super Marathons, but when I blew through the sidewall of a Vittoria due to my brakes being poorly adjusted, the plain Marathon was what my shop had in stock. Now I'm glad that's what they had in stock!