Julii are the easiest to expand quickly with, in my opinion, because of all of the small-sized territories to the north and west which are fairly easy to take over quickly (until you get to Germania, and even they're not that hard to beat). However, the make the least money, because those regions are poorer and many are landlocked.

The Brutii allow you to become very, very rich quickly, and expand to a good number of territories fairly quickly, but have the problem of facing some of the harder factions to beat early in the game with hastati and principes (Macedon, Thrace, and Greece with their phalanxes, and some other units, such as the Thracian falxmen).

The Scipii are more middle of the road for money, but have the problem of being very slow at taking territory, because there's only have seven territories to the south and west of them, not including Caralis, Petra, and Spain, and those territories. Although having fairly easy opposition, assuming Carthage doesn't have a lot of elephants, the territories you have to conquer are huge, and it takes a long time to get an army from Carthage to Dmmidi (or whatever it's called), and then back to take on your next logical opponent, Egypt, before it gets too strong.