By optimistic, I assume you mean the most balanced and versatile.
I'd have to say that Brick break is the most optimistic Fighting-type attack. It has reliable power (With a decent Attack stat, it packs just enough power to OHKO most variations of Aggron), along with a healthy PP capacity.
It also has the useful utility of breaking through Reflect and Light screen, which is very useful.
Lastly, Brick Break is a TM, and has incredibly wide spread (If a Pokemon has hands or some kind of blunt-headed limb or tail, it can probably learn Brick break).