The Roman factions for sure. Just the fact that there are three of them is amazing.... You only have to worry about a couple fronts because you are almost guaranteed that ur allies won't backstab you. There infantry is 2nd to none with disciplined foot soldiers able to adopt to many situations, excellent calvery that is fast, well armored, and nicely armed. They also have many pieces of artillery such as repeating ballistas, onagers, and heavy onagers. There tech tree is also very vast, and their temple bonuses are usually very good. I think they are the best because of the game name... "Rome Total War"