This is one of my favorite stories from Livy.

The way he tells it (minus the details) Brennus and a bunch of other roughy Gauls marched into N. Italy where they scattered the Etruscans around the River Po. They then moved down towards Rome where they scattered the Romans about 12 miles north of the city. Then he reached Rome, herself.

Livy also tells us of how the Gauls thought the senators were statues, and when one of them grabbed the beard of a senior senator, Marcus Papirus, and the senator slapped him on the hand. The Gaul, not wanting to put up with this old-timer's shenanigans, gave the guy a whipping he would never forget, and killed him. Apparently they went to work on all of the senators and continued to loot and steal (don't even say that you wouldn't do the same thing).

Once the Gauls defeated the Romans and got control of Capitoline hill, Brennus had Quintus Sulpicius weigh out the gold that he would take from them. When one of the witnesses cried injustice and that the Celts were taking their ransom unfairly, Brennus threw his sword on the scale and yelled out, "Vae Victus", or "woe to the defeated".

The Celts, happy with their ransom, withdrew from the city and left the Romans unmolested.

In short, the answer is Brennus.