Flask of Poison

One coating at a time. Drinking a different flask wipes whatever you had on, so there is no stacking poison with fire. The compensation is that the effect rides out your death, which no other buff category does, so a failed boss attempt does not cost you the flask on top of everything else.

It applies to melee swings and to anything a melee weapon throws, so spears, boomerangs and flails all carry it out to range rather than only landing on contact. Whips were folded in later, which quietly made the whole flask category part of the summoner toolkit as well.

Poison is the weakest of the line and the damage it ticks for is minor. Where it earns its place is on slow heavy weapons, since the gap between swings is exactly when a damage over time effect is doing work for free. If you have the materials for one of the later flasks, that is a straight upgrade and there is no reason to hesitate.