For me it would be an Antminer L9+ or a D3, but the difficulity is hard to get one of it because withing hours stocks are sold out.
the L9 (don't know this ASIC) and D3 aren't sha256d asic's. The OP was asking about bitcoin miners, not altcoin miners.
@OP: at this point in time, the antminer S9 is the sha256d ASIC with the lowests J/Gh on the market. However, you still need pretty cheap electricity in order to mine profitably, and bitmain only offers 90 of warrantee, and the S9 is known to break easily.
If you go for canaan products, they are known to be a bit sturdier, and their support dept is known to be a little bit more user-friendly, but their best product is not as efficient as the S9.