Argo HP w/17870 li-ion cell and upgraded emitter is my favorite work headlamp. And backup headlamp to Stenlight for caving.
here's the original Argo HP mod post with detailed how to instructions.
http://www.candlepowerforums.com/vb/showthread.php?t= beam pattern was extremely tight even after shaving quite a bit of material off reflector to make switch from side emitter to high dome Sbin luxeon. beam pattern ended up extremely tight.
have not done Cree or P4 conversion yet, but my guess is Cree would more closely mate to Argo HP reflector. My Sbin conversion works so well. have not needed to open backup. probably will purchase another Argo HP to try Cree mod.
if you want to talk bright!
NiteRider Storm HID and Lights & Motion ARC HID headlamps will put out more lumens than any other headlamps that I'm aware of. both come with li-ion packs that will run for 6 to 10 hours+ depending upon pack chosen. (up to 8x 18650 pack)
Stenlight currently is the best of it's class caving headlamp available. uses 2x 18650 li-ion pack or several other pack configs.
For Argo HP ($30 + $10 cree + 17670) to be mentioned in same league as it's $300+ brothers shown below.....
Argo HP mod is a super simple, durable/water resistant, low cost mod that's small in size while giving 10+ hours runtime on high (30+ hrs low) for me the perfect work headlamp!