Who do you think is the hero of the play, Julius Caesar or Brutus?

It's possible to argue that Brutus fits the usual description of a tragic hero, which is someone basically decent brought low by a fatal flaw. Everyone agrees that Brutus is an honorable man; even Mark Antony thinks so. He cares deeply about Rome and wants to preserve and protect what he sees as its most cherished and ancient traditions. Above all, Brutus wants to defend the venerable old Roman constitution, encapsulating as it does the tradition of republican liberty he values so much. To that end, he looks upon Caesar's political ambitions as a serious threat to the existing polity. He's absolutely convinced that Caesar wants to make himself king, eventually turning himself into an outright tyrant who will destroy a system of government that has endured for centuries.
Brutus's fatal flaw is that he's too naive, too trusting. He automatically assumes that all the other conspirators have joined the assassination plot out of similarly disinterested motives as himself. He's so blinded by his high-minded sense of duty that he simply cannot comprehend that other people want to kill Caesar for purely selfish reasons. This enables the likes of Cassius to use Brutus as some sort of respectable figurehead to give their assassination plot and subsequent grab for power a facade of noble respectability.