So as the title says:
Prophecies
Factions
Nightfall
EoTN ( an EX pack but included nonetheless)
I think for me it's Prophecies with Factions a close second.
The original was long enough to "forget" the jungle after running a few toons throughout the game without losing anything in the story so to speak. Whenever I go through over again the jungle is enjoyable.
The missions are fairly immersive and no time limits attached to bonus is a nice feature. I recently brought a toon out of Pre with the intention of using just henchmen to go through the game. I gave up and recruited heroes somewhere in the jungle. Alesia is possibly the worst henchmen with her healing( I was playing a smite monk, that seemed to be lacking any real damage potential too) Maybe I'll revisit with the mesmer I have in Pre in the future.
Factions was the least enjoyable campaign when I played through until 2012. I found the running around town quests to be a bit tedious going back and forth between all the people for quests. ( Of course 95% are unnecessary for the flow of the story line).
The addition of timed missions for bonus brought tactics and routes into play. The campaign is short enough to go through quickly. The last 3 missions move so quickly I have to finish out the game when I get to that point.
Favorite mission would be Thunderhead Keep. Favorite place to hangout is Grenth's/ SF.
I still have a number of toons that are working through all the campaigns.
One of the more interesting components before GW2 came out was PUG's, It's a shame still cant go through the game that way.
I could join a different guild, I guess, I created mine in 2006 and really not sure I'd want to lose that aspect of the game. The head guild of the Alliance of my guild is in was created shortly after launch. I see one old alliance mate log in maybe every 6 months but thats it