That really depends on so many factors. Take a 7950, the power consumption while mining can easily vary from 180W or less to 250W+ with the core and memory overclocked like crazy. The secret of mining is to get a good ratio. There is no point pushing a card to, for example, to 1.3V if you only get 5% more hashes vs running at 1.1V or less. Because the extra voltage could use up 20% more power.

ATM, possibly the most efficient card is actually the 7790 (GCN 1.2) but while it's great to get 300MH/s with only 60-80W, that's not a great hashrate these days. Great hash/watt though.