Question

in the lesson of Julius Caesar
whom does Antony refers to as "the choice and master spirits of his age"

Answer 1

he refers the conspirators

Answer 2

In the lesson of Julius Caesar Mark Antony addresses the words "the choice and master spirits of his age", to the plotters who killed Caesar.

Mark Antony utters those aforementioned words as they have killed the mighty Caesar, that Caesar who accomplished many great tasks and conquered great empires.

And now the conspirators are now liable for the working of Rome. Now they have the choice along with the power. In addition, they are the masters of what to do next in Rome.

Antony requests these conspirators to kill him too. This is because of his realization that that was the fittest hour and apt place to being killed along with his master and friend, the great Julius Caesar.