Flask of Ichor

Ichor cuts enemy defense while it is on them. That is a flat subtraction, so it helps every source of damage you own and not only the weapon that applied it, and it stacks on top of everything else you are running. Keep hitting at a reasonable pace and the debuff never falls off, which puts this ahead of every other flask on total output.

Slow, heavy weapons are the exception. If you swing once every couple of seconds, a flask that adds damage on hit does more for you than one that shaves defense you are not exploiting often enough to notice.

It coats whips as well as melee weapons. That matters for summoners, since tagging a target with a whip means the defense cut applies to every minion hit that lands afterward. Flasks never stack with one another, so drinking a different one wipes this off.

The weapon glows while the buff runs, brightly enough to replace a torch. On a yoyo or a whip that means exploring with the light out ahead of you and never swapping items to see.