What version of the game guild wars is the most fun? Original, Nightfall, Factions, Eye of the north-expansion?

There can be no real answer to this because the first three games are all interconnected and the final one is an expansion. If you are asking however, which ones provide the most synergy (min 2) then I would have to say original and factions. Factions give you AB battles and alliance access while original gives you a lot of basic skills you will need. Nightfall and EotN are both mainly skill adders.