Lightning Spear in (at least) early game Dark Souls. I recently restarted after being stuck at Sen's Fortress for two years and having a screwed up character. I went in with a plan (spellsword with faith) and decided to use guides only when I needed to find things necessary for my build. Pretty much everything else I've decided to take my time on and find out for myself.
Once I got Lightning Spear, I started tearing through absolutely everything. I killed Lautrec no problem. Havel fell quickly, and now I'm ready to absolutely decimate the Gargoyles. I'm having so much fun!
EDIT: Moonlight Butterfly was an absolute joke with this miracle, which is a shame considering how beautiful and awesome it is!