Michelins, but I have to special order or order them online. When I was looking for ultralight 70g tubes for the race wheels on the road bike, I ran into several comments like "nothing holds air like a Michelin". Their Aircomp ultralights actually maintain pressure as well as the heavier QPB tubes I run in the commuters. I may just throw some Airstops in the tour bike next time I take it out for a long weekend.
Also Vredestein. They seem like a very tough long lasting road tube for 100g. These are the ones I use in the training wheels for the road bike and when I've got 700x25-28c road tires mounted on the cross bike. I've never had a torn valve stem or anything of the sort. I don't believe I've ever seen them in a legit cyclocross size though. I would probably pick some up if I did.
Continental does make tubes in a 700x28-47 size.
http://www.bikemine.com/proddisplay.asp?prod=2087
I've had good luck with a few Conti's in the mountain bikes but there isn't anybody in town that stocks them and they don't "feel" quite as tough as the Vredesteins which are generally available at the nearest shop.