As pointed out by previous poster Factotum, there is no universal answer.
That said, in my experience the cheapest countries are those with "artificial" economies, i.e., price levels set by the government and unrelated to world or market prices. That is why Venezuela has gasoline for 6¢/gallon (depending on the exchange rate) and other countries subsidize bread to sell for pennies a loaf.
My nominations are Iran and Burma where a traveler can be perfectly comfortable for a few dollars a day. Taxi rides are under a dollar, restaurant meals a couple of dollars, and a decent (air-con, decent plumbing, etc.) hotel room $10-20. If you want to be cheap, you can ride the bus and eat street food for a few cents. A less-isolated but slightly more expensive country is Syria.
If you want to stay in international-class hotels, be prepared to pay commensurate prices, for which the Big Mac index is a pretty reliable indicator.