Out of the 3 I would agree Factions is really easy to max out level-wise. You could get the expansion, Eye of the North, which provides more PvE and is picks up where Prophecies leaves off, but doesn't have any new PvP content as far as I know. Factions features Alliance Battles which is very fun, while Nightfall provides a deep storyline, and you will also receive heroes which can be used to Hero Battles.