I think it's the Maris otter. I consistently get 77-79% efficiency with 2-row based bills and I've made about forty all grain batches. So far I've made 6 Maris Otter batches. I've gotten 92, 94, 71, 65, 64, 69% with Maris otter. I've been avoiding MO because it is unpredictable for me. The size of the grain is larger so it seems to crush more fine because of this. But as you can see I get far below good efficiencies sometime as well.