I would suggest an Arc head on a D cell body, nice and bright and it would last forever. Now if we could only get someone to make that adapter.
Personally I would take a 4 D or 4 C cell lantern, take out the regular lamp, short one of the battery contacts (so it used 3 cells) and replace the lamp with a LED. I have one and I don't think I will have to change the battery any time soon.