There's no doubt that Dionysus is the protagonist of The Bacchae. He's the center around which the play revolves. He's the one who disturbs the world of the play by bringing his new religion home to Thebes. Most importantly, Dionysus is the one who drives the action of the play. He shows up at the beginning, tells us he's going to punish all the foolish mortals who are defying him, and that's just what he does. Dionysus is a great example of a truly active protagonist.
Though Dionysus is the protagonist, it may be kind of hard to think of him as a "good guy." A lot of people try to pigeonhole the role of protagonist in this way, but very often it just doesn't apply. Dionysus does a lot of messed up stuff during the play and all for his personal pride. Though there is a hey-lets-frolic-in-the-woods side to him, there's also a hey-lets-rip-people-apart side. Such un-heroic heroes are pretty typical of Euripides. Check out our guide to Medea if you're looking for another example.