I have a Jalapeno sitting next to me doing 5.5GH/s for 45watts.
That said, get any voltage unlocked 7950 or 7970. They can reach some impressive numbers for a GPU. However, you want to look at profitability, not most energy efficient. Here's what I mean:
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I can undervolt to increase my MHs/watt to 2-2.5x stock, but I actually lose profitability.