Before Christianity who was the ultimate authority in the Roman Empire

if were talking about religion... then..

Religio Romana

The Religio Romana (literally, the "Roman Religion") constituted the major religion of the city in antiquity. The first gods held sacred by the Romans were Jupiter, the highest, and Mars, the god of war, and father of Rome's twin founders, Romulus and Remus, according to tradition.

 Over time, the Christian church and faith grew more organized. In 313 AD, the Emperor Constantine issued the Edict of Milan, which accepted Christianity: 10 years later, it had become the official religion of the Roman Empire.

Religio Romana

Explanation:

before Christianity had been introduced to the romans and Constantine made it the official religion of Rome, The head of roman religion would have been the god Jupiter.