6 NiMh C cells will get you the baddest ROP around, with over 1 hour of runtime. You need a 5Mega aluminum reflector (I'd recommend LOP or MOP) and a glass window. I purchase mine in packs of 5 from KD, for only $3.
In terms of least to most bright bulbs for 6 NiMh cells it goes:
Maglite 5/6 cell bulb-->Sears 6v lantern bulb-->ROP Low-->WA1111-->ROP High
I say go for broke with the ROP high bulb. There's nothing that can compare to 1k incandescent lumens.
The best NiMh C cells are the 4.5k mAh Accupower's from Thomasdistributing.com, which cost ~$15 a pair. They're LSD so it'll always be ready to go, even one year after the last charge. If you want to spend a bit less on batteries, all-battery.com has 5k mAh cells for $4.50 a piece.