I frequently refer to two studies by led by Robert Ross that look at aerobic exercise and weight loss. For me, these two studies are benchmarks that can used to compare other exercise programs and weight loss. So far, I haven't found anything that really comes close to these two studies as far as total weight loss. One note of caution: the subjects did lose a small amount of lean mass in these studies, but this could have easily been prevented by performing a small amount of weightlifting such as one session per week. In other words, I don't think a program of solely aerobic exercise is the best, but I think these two studies show it is very effective.
Here are the two studies in pdf format. They are both freely available online, so I think it's okay for me to put them up here: Study 1(pdf) and Study 2(pdf).