Whats the best roman faction in RTW?

[Myth]

Yes starting positions are vital. They are:

1. Brutii: Easy access to the lands of GOLD aka. the Peloponnese peninsula. Sparta, Athens and Corinth are top tier cities so long as you don't slaughter their populace. Thessalonica, Buzantium, Halicarnassus and Rhodes are a stone's throw away and will secure your gold income and troop production permanently.
2. Scipii: They can sail towards the ladns of gold fairly early as well, but they cannot spare the troops, least they loose the battle for Syracusae. However, they can get the nice island towns, Carthage and Cordoba fairly early, and then stomp on the twin Egyptian cities. Sea trade will be chasing down the Brutii though, unless you count the temples. More on that later.
3. Julii: They do get good starting stacks, with both Roman Archers and Triarii. However, their boats can take them only towards the small islands in the Mediterranean and the desolate Gaulish coast. They could try beating the Scipii to Spain, but that will leave their Italian cities exposed to the Gauls early on.

So the Brutii are top dogs as far as starting position is concerned.

Temples:

1. Brutii: They get the overpowered temple to Mars, which grants experience and at the last level, weapon/armour upgrades as well. Experience is very powerful in RTW as it grants not only morale bonuses, but direct stat boosts to your units as well. Combined with a high start general these just skyrocket. The temple to Mercury can make the Brutii so rich they actually end up wondering what to do with their denarii, as a smart player always keeps his treasury below 50k to avoid the nasty corruption traits for the good ol' family. I choose not to make them anymore, simply because i can't spend the money even if i fill all the building ques/recruitment ques. If for some reason, a Brutii player finds himself in need of gold, these trade temples can make their lucrative provinces in to literal goldmines.

2. Scipii: decent equipment temples, one that grants a marginally useful boat (at this point in the game you should own the sea battles simply due to numbers anyway) and the third one is meh.

3. Julii: A fertility temple which is marginally useful early on. A law temple which is OK for making good governors and useless from a military perspective. The third one is a no-no unless you plan to never ever spend the night with a general in that city.

So in both scenarios the Julii are the underdogs. I've never actually played trough the civil war, it's simply too tedious and my laptop is ever so slow. However, once you take the two Julii cities on the Italian peninsula, coupled with Mediolanum and the other large northern town, you can just leave them be IMO. The Scipii hold better settlements anyway.