Which is the best weapon buff?

Short answer: There is none.

Long answer:

In the early game, Rockbiter and Flametongue both have their uses.

Rockbiter is a good to use when:

* fighting low armour mobs. AC is taken into account when rockbiter damage is calculated
* as a main or secondary tank
* fire/cold resist mobs
* as the base default buff when soloing in the early game.


Flame Tongue is good to use when:

* fighting high armour mobs, as the fire damage is constant despite AC
* against fire-vulnerable mobs
* if you have +spell damage equipment


Frostband is good to use when:

* fighting mobs that run. Yeah, it won't proc *all* the time, but it should save a chase or a shock spell to make it worth the while
* frost-vulnerable mobs