Without question HC-110. But it is rather thick so my method of dealing with that and not wasting concentrate is as follows:
I would get a small 50ML graduate and two 1L graduates. I pour the measured syrupy HC-110 into the 50ML graduate, then pour the correct amount of tempered water into one of the 1L graduates. I then take the empty 1L graduate and pour a couple ounces of the tempered water to pour the HC-110 concentrate into so it starts to dissolve right away and is not stuck at the bottom. I then take the rest of the water in the 1L graduate and keep pouring it into the 50ML graduate that still has some residual concentrate into it until I have basically cleaned out the small graduate and then pour the remaining water into the developer graduate...
Basically I use HC-110 at dilution B which is 1+31 and it lasts a long time and the negs come out really great. It has pretty much replaced D76 for me.