Most efficient gears for bicycle traveling uphill?

The number of gears aren't as important as the gear range. You want a low enough gear that you can comfortably spin the pedals at 80-90 rpm while climbing. If you can't do that on hills, then you need lower gears. So the answer depends on how strong you are and how steep the hills are. Most road cyclists get by just fine with double chainring setups, regardless of how many gears they have on the back.