I think the real question might be, "efficient under what conditions."
The problem with the Pocket Rocket, Crux, etc. is that they don't perform well in wind. You can go through a lot of fuel on a windy day.
A Jetboil PCS is more efficient in its heat transfer and works better than most in wind, but it's a heavier stove.
The stove that's really efficient in the wind is the MSR Reactor, but I don't think anyone has ever called the Reactor light.
In terms of lightest weight, I think it would be important to consider how many people for how many days and under what condtions.
If you've got a larger group or a longer trip, you actually might save weight by carrying a heavier but more efficent stove. The more efficent stove might save you from having to carry a second canister, resulting in weight savings.
From the charts I've seen online, there's kind of a "sweet spot" where the trip and number of people is such that you burn most of the canister you bring but you can get by on a single canister. If the trip is long enough that you have to carry a second canister, then your weight savings tend to evaporate.
If you're going on a trip and you know it's going to be windy, again a heavier but more efficient stove might save you weight simply because you don't have to carry add'l fuel.
There are ways to make a canister stove more efficient, namely a wind screen. Now, unless you've got a remote canister stove, then pretty much every manufacturer says (essentially) "if you use a windscreen, you're going to cause an explosion." It's true that an over heated canister will explode, but canisters aren't quite that touchy. If you pay close attention and you use a heat shield between the canister and the flame, you can use a wind screen. You just have to frequently touch the canister. If it's to hot to touch, then you need to take action quickly to prevent the canister from heating further.
There's information about heat (radiation) shields on this site. There's quite a lot of information at www.bushwalking.org.au on heat shields and wind screens as well as a wealth of tips, tricks, and general stove efficiency information. Note: bushwalking.org.au is a bit prejudiced in favor of canister stoves