I agree that Rival Wings is the one you're most likely to have fun in right away. Frontline is more of an acquired taste - it becomes fun once you get good at retreating with the alliances so the enemy teams don't mob you, and when you understand all the objectives and work on them, and when you can hold your own against 1-3 other players without dying. Crystalline Conflict is fun for a lot of people too just because it's made a dynamic experience due to players, and I like direct PvP a lot, which it is much closer to being than the others.