Lightest one are currently the NCR18650B at 3400mAH. But they are very expensive.
You can get the cheaper NCR18650A at 3100mAH (safer and cheaper), or even the NCR18650BD at 3200mAH (stronger and more expensive)
Let say you get the high discharge NCR18650BD, 20 cells would give you 36V6.4AH...