It can be quite confusing when you buy all of the Guild Wars 1 campaigns to figure out which one to play first. I hope this list will help. Note that all campaigns are pretty much stand alone so you don't need to play them in chronological order.
Please note that you need to have atleast have bought a campaign before you can play the expansion Eye of The North.
This is the third campaign but is also the easiest to pick up and it introduces a very important feature, heroes! Those are completely customisable AI companions which will stay with you wherever you go (including all the other campaigns).
Second campaign. It's not recommended as the first one to play because it's pretty hard and it requires you to have some grasp on the mechanics. It's also one of the shortest ones because it focuses more on PVP than PVE. It does have a kickass oriental theme though!
Guild Wars Prophecies or also known as Guild Wars Game of the Year edition on Steam. It's the oldest and longest campaign and is a pretty slow burn. It can be quite hard to get into because you don't have any access to heroes. Provided you already have heroes, you can pretty much slug through the slow parts pretty fast.
I would play this one last. It has some cool endgame PVE content, looks great and requires you to have an indepth understanding of the mechanics. Great way to end the series!
After you have completed all the campaigns, you can do "Guild Wars Beyond" content which is a series of quests that set up the story for Guild Wars 2.
