For beginners to RTW, I recommend the Romani, Qarthadastim, or Sweboz, as they are the easiest factions, featuring a pinch of everything and familiarity (coupled with good starting position and economy), one of the best economies in the game (tied with Epeiros/Makedonia), or the best starting position in the game (respectively). The Ptolemaioi are also a decent choice here, though you'll have to fight the Gray Death , and the Getai are another good option here.

For war veterans who need to adjust to EB's demands, I highly recommend the Aedui/Arverni (showcases most of EB's new principles, especially high cost and tough foes). If "barbarians" aren't your thing, Baktria is an excellent choice (you fight two completely different enemies at once).

For tough cookies, try the nomads, who are much more prominent than in RTW and have (arguably) changed the most. Their economies are terrible, so you'll need skill with the few troops you can afford unless you don't mind super-deep debt; the Casse offer a similar experience. If those simply aren't right, try playing as the ever-present enemy of all, the Arche Seleukeia.

If you think those sound too easy, and you think you're an expert, try Hayasadan, Pontos, or the Saka Rauka, who all feature endless waves of gray banners, weak starting economies, and high risk-high reward options everywhere. Every battle counts in these, and everything is on the line, at all times.