Costco. Not because the air mattresses are great, but they are decently priced, and then when they get a leak every 6 months, Costco's excellent customer support will replace it for you. We've gone through 3 in 18 months. On the first, kids were probably being too rough with it, but the second time around, we took good care, didn't have fatties sleeping on it, and kids didn't play around on it, and that leaked too.
If you can be talked out of an air mattress though, a nice foam pad and a decent sleeping bag are superior IMO.