Some people on this forum have said they're comfortable with 32 or 37mm. But those who say they've passed in very rough areas said they couldn't survive even with a 38mm. So if I was going with a 700x42 or 26x1.75, I would consider the Panaracer Tourguard Plus Universal Cycles -- Panaracer TourGuard Plus Tire available in 700x42cc and 26x1.75 Universal Cycles -- Panaracer TourGuard Plus Tire
There's also the Michelin Protek Cross Max apparently available in 700x42 on bike24.com: Bike24 - Michelin Protek Cross Max 28 inch Wired Tire

*Disclaimer, I haven't tried these yet but just guessing based on size and tread patterns. They look "all-around" kind of tires instead of very slick or very knobby.

Of course, what's implied here is that you have a bike that can take these size tires.

There's also a Michelin Cross 37-622 here: http://www.xxcycle.com/michelin-prot...e-28-w,,en.php and in Canada http://cyclepath.ca/shop/product_inf...37a796e7510c1a

Ha, I found one on amazon in France even though I'm in Canada. (Michelin Cross 42-622) http://www.amazon.fr/Michelin-Protek...otek+Cross+Max