Firecracker

The whip itself does very little. What it does is paint one enemy, and the next time a minion or sentry connects with that target the hit detonates for several times its normal value and splashes everything standing near it. Only one enemy holds a mark at a time, and each mark fires once. So the rhythm of playing it is whip, let a minion land, whip again. On a single target with a busy minion, that loop is where nearly all of your damage comes from.

Since the payout multiplies whatever hit set it off, damage per individual hit is everything and the minion's attack speed barely matters. Slow, heavy summons are ideal. Abigail's Flower, the Desert Tiger Staff, and the Stardust Dragon Staff all concentrate their output into big hits and all get stronger the more slots you feed them, which compounds with this whip better than anything else does. The Sanguine Staff is the easy early Hardmode pick. Swarms of fast, weak hitters waste it, because they consume the mark with their smallest possible hit.

Spinal Tap and Snapthorn leave a lingering bonus on the target that keeps working while you do something else. This one leaves nothing. Stop whipping and your damage falls off at once, which is demanding in phases where you would rather be reading the boss than keeping a rhythm. Against the Cool Whip it wins clearly while you are actively attacking and loses when you are not. The Starcrash does something comparable much later if you like the pattern.

The explosion cannot hit the enemy carrying the mark, only the ones around it. On a lone boss you get the multiplied hit and nothing more, which is still the main event, but it means the weapon reads far stronger in a group than in a duel. The swing itself also loses a chunk of damage on each extra target it sweeps through, so do not think of the whip as your crowd tool. The mark is the tool.

It can drop the moment you beat the Wall of Flesh, which makes it potentially the first whip you ever hold, and it stays relevant much longer than that start suggests. Legendary is the default. Light is worth a thought if your minions strike often enough to consume marks the instant you apply them, since at that point applying the next one sooner is worth more than hitting harder.