Newer KWA guns have pretty good efficiency,like the Glocks,Sigs and M9 PTP. Not sure on their 1911s,they don't have such a good reputation,from what I've gathered on r/airsoft and a few other forums.
Maruis,as usual,have great efficiency,bit better than KWA due to plastics.
KJW Co2 Hi-capas have very good efficiency also,but only on Co2. They have awful efficiency and cool-down on green gas. I used my BattleBro's KJW Hi capa and got around 4 and a half mags worth of BBs,with a 24 round mag,in warm weather.
The WE G39C has pretty good efficiency and not much cool down. It's a shame that the hop on it sucks.