Prophecies is the story about what happened in Tyria (the continent) 250 years before GW2. You explore much of the same areas and a lot of historical people mentioned in GW2 appear in this. It's the oldest game and it shows. It's slow paced, the missions are spread over a huge landmass and you level very slowly. Extremely beginner friendly, almost to a fault.
It's the core game that has most connections with GW2 and if you only wanna get one of the campaigns, this is the one to pick for lore reasons.
Factions is about Cantha and far removed from pretty much everyting in GW2 (so far). It's extremely fast paced, you level quickly and the missions are pretty much just stacked on top of each other. It has two very fun professions that you can't access if you start in the other campaigns. Shiro is the big bad in this one so for Rev lore it's sorta fun to see what he did and so on. Least important one tho, imho.
Nightfall is also quite "unimportant" to the GW2 lore, but it's (imho) the best campaign. It's beginner friendly but not as extreme as Prophecies and it has good story/mission pacing that is a bit slower than turbo mode Factions. It also introduced "Heroes". Before Nightfall you could hire "dumb" AI helpers that had a set skill bar and behaved a bit like Necromancer minions. Heroes are helpers you get that you can customize. Skill sets, weapons and behaviour can be set, and you can tell them where to go and who to attack. Like more advanced ranger pets. You can take them with you to play the other campaigns, but you have to have Nightfall to get them (or EotN, but I'll get to that). This one also has two unique professions that I really like and the setting is amazing.
Eye of the North is the bridge between GW1 and GW2 and completely essential. It's an expansion that works with any of the three core games but the story and setting are pretty much only connected to Prophecies. It also uses Heroes instead of the dumb helpers so you're not missing out completely if you're skipping Nightfall, you just get them much later.
A lot of people dislike the gameplay of GW1 so it's hard to all out recommend getting everything. I would recommend getting Prophecies, playing until you get to Lion's Arch and then decide if you liked that experience or not. If you did, get either Nightfall or both Nightfall and Factions, plus the very much essential EotN and start amew in Nightfall to get the Heroes. This character can later jump to Prophecies where the story picks up in Lion's Arch. That way you get to see the whole story of Proph but with the extra class choices Nighfall gives you and the Heroes.
If you didn't like it at all, stop playing (duh
) and i you find it just bearable, get only EotN and at least finish Proph and EotN.
Puh, I hope any of that made sense!