It depends.
If you want the highest performance and efficiency, the Bitmain Antminer S9 models - but they are widely known to have reliability issues.
If you are more worried about reliability, the Caanan Avalon 741 has a very good track record, at the price of a little lower efficiency and lower performance than the S9.
EBang BW.Com and Hotmine don't have as many units in the field in the hands of non-large-mines so their track record is a lot more "unknown" except for the large farms that have lots of their units.
Innosilicon dropped out of making SHA256 miners entirely, but the ones they DID make were rock-solid reliable - it's a major bummer that they didn't move into the 14/16nm generation.
Many thanks.