if you are sticking to 2 cell size, you have basically 2 options:
1. Run a Surefire P61 on a pair of standard lithium primary cells.
2. Run a LumensFactory HO-9 on a pair of AWs rechargeable protected li-ion RCR123 cells.
Configuration 2 should only be used in short increments, it's hard on the cells, cells need to be monitored frequently for safety.
Both of these options are about the pentacle of incandescent output in this form factor, Both are probably comparatively bright, the HO-9 setup will probably start off brighter, with good quality lithium primaries the P61 will hold a steadier output through the run. The HO-9 will also be a tighter beam. In the grand scheme of things they won't be dramatically different in total lumens... But if you top off the cells in configuration "2" frequently (only using like the top 20% of the cells capacity, like 5 minutes of runtime before topping off) then it will be brighter than the P61 in use. Runtime for either config is about 20 minutes.