The front derailleur shifts with the left shifter (STI, Ergo), and the rear derailleur shifts with the right shifter. To be consistent with this, the left brake lever (which is part of the FD shift lever) should actuate the front brake, and the right brake lever (which is part of the RD shift lever) should actuate the rear brake.

This should also be consistent with what predominates your geographic region (US, continental EU): front brake on left, rear brake on right.

If you were in the UK or Ireland, you could reasonably put it the other way around, front brake on right, rear brake on left, as traditionally this has been the setup. But with STI/Ergo levers, even this is changing.