Flask of Fire

One sip and every melee swing and whip crack sets things alight, including the projectiles your melee weapons throw. The burn itself is small. What it buys you is damage that keeps ticking while you reposition, plus enemies that glow while they burn.

That second part is the underrated one. Pushing through a cave in early Hardmode while setting fire to everything you touch gives you a moving light source for free. The buff runs long and survives death, so a single flask usually covers an entire trip underground.

Hellstone is the sticking point. It used to be the one flask a world could permanently run out of, and even now brewing a stack means a trip to the Underworld first. If a fight has a stronger flask available, take that instead: only one flask effect can be active, and drinking a new one wipes the old.