I would say Nightfall, as you can get heroes which let you equip them with spells and gears that you want. The story is pretty good too, though if you really want to follow the storyline, it should be in this order : Prophecies, Factions, Nightfall and lastly Eye of the North.

For leveling speed, Factions > Nightfall >> Prophecies.

Bear in mind the Eye of the North requires at least 1 of the above game to be bought 1st as it's an expansion, not a standalone game.

im playing this slow2 elem/ranger lvl 11 only...n playing alone