personally i would go with Prophecies, Factions, Nightfall, Eye Of The North.
Even thought eye of the north is the newest doesnt mean its the most populated, i have all gw games and the campaign i play on the most is Prophecies.
Nightfall and Factions just give you extra classes and a new storyline. Yes the stories are cool in those and Factions has the alliance battles and such, but in prophecies you get the feel for the game and can test out the basic classes before you go to the advanced ones (i.e. Dervish, Paragon, Ritualist and Assassin)
But playing prophecies is not only a easy start for most people, it is also a good way to get people to join a guild
