What does Antony mean when he refers to Brutus as “Caesar’s angel”?

In Shakespeare’s tragedy Julius Caesar, Brutus is the last among the group of Romans who together murder Caesar by stabbing him. Marc Antony did not participate in the murder. Afterwards, he speaks to the Romans assembled in the street (act 3, scene 2). In several speeches, he condemns the murderers, while still claiming to understand their stated motives of opposing Caesar’s tyranny.
When speaking of Brutus, however, Antony is especially harsh. Showing the crowd Caesar’s robe, he points out all the tears that the knife thrusts created and points especially to one rip that he says Brutus made:

Through this the well-beloved Brutus stabb'd;
And as he pluck'd his cursed steel away,
Mark how the blood of Caesar follow'd it

The line quoted in your question immediately follows:

For Brutus, as you know, was Caesar's angel:
Judge, O you gods, how dearly Caesar loved him!
This was the most unkindest cut of all;
For when the noble Caesar saw him stab,
Ingratitude, more strong than traitors' arms,
Quite vanquish'd him: then burst his mighty heart

By “angel,” Antony means, as the next line makes clear, beloved. Telling the people that it was personal betrayal and a broken heart that finally killed Caesar, Antony not only pins the blame squarely on Brutus, but also dismisses the validity of patriotic motivation.