Probably the Antminer S5.  It is now not uneconomic for most people if they have to pay for the electricity.  You may be able to get an S5 plus psu for $150-180 USD.

The exact answer will depend on what miners are available in your area.   You should be researching your local area as the cost of shipping is likely to be excessive if you cannot pick up the miners yourself.  

You also need to make certain the PSUs are included in the purchase price.  The PSU is likely to represent about half of the miner cost.

Currently you would make about 6 dollars a week per S5 with free electricity.

To work out your ROI use a calculator such as http://www.coinwarz.com/calculators/bitcoin-mining-calculator

Also see https://bitcointalk.org/index.php?board=75.0