You can define “best” and “worst” as you wish.

In my opinion, the Federation and Star Fleet are a beacon of idealism and corruption. Who’re fully willing to let entire peoples die for their prime directive, who’re entirely unable to wrap their heads around the concept of military corruption. These two complaints can’t be found within the Borg Collective, The Borg don’t have hierarchies (even with the “Queen” every drone is equal to the whole of the collective), and thus they are immune to corruption. Their view of the world is strictly dialectical, their galactic quest is void of idealism. Thus, I would say the Borg Collective are the best faction (at least in the TNG era)

For worst, one need look no further than the Romulan Star Empire, Their Imperial government is everything that’s wrong with both the Federation and the Klingon Empire. They’re nothing more than lying, cheating xenophobes with an authority kink. Heck 'em.