Elecrow also does $1 batches for all customers but shipping kills it.
It's impossible to generalise and say one is always the cheapest, it depends on the count, size, layers, material, options, assembly or not, shipping method, intro offers, and so forth. Pcbshopper can give a starting point but one has to put the info into their web forms to get a quote. But there are a few already mentioned that tend to be cheap for hobbyists.