I've played ffxi since the beginning when it was first released on ps2 in NA.
The whole reason I've been leveling up/gearing my character is to play through the story lines, and I've watched the stories evolve a huge deal. When you compare the newer content with the first few missions/quests, it's like night and day.
When I joined and RotZ was the newest (and only) expansion, I thought the story was a bit weak, but I still really enjoyed the battles and felt accomplishment at getting Sky access. It wasn't until CoP came out that I started to actually remember the characters in the story, and I remember being very impressed by how I really wanted to see what would happen next in the story, and caring what happened to Prishe/Ulmia/Tenzen/Louverance. I felt like there were some unexplained things in the plot, but the characters/music/dialog were very good as a whole.
I thought COP was going to be as good as it would get for ffxi's story-telling. I played through ToAU's story and, although I felt it was ok, I didn't feel as big of an attachment to its characters. The music and some of the cutscenes were excellent, but I wish it had more supporting cast, like how Cait Sith became a important recurring character halfway into the missions. I really liked ToAU's other content, so I was satisfied with the expansion.
But just the past week or so, I've been powering through WotG's storyline. I had just barely began the questline when it was released, but didn't get very far and I was disinterested to continue, until they released Cait Sith and required wotg to be beaten to get it.
And man, I was -very- impressed by the story. I found myself attached to all of the characters, even the villain, and when I finally beat all the missions, I got that same empty feeling you get when you finish a really good book/movie - when you don't want the story to be over. I honestly was looking at completing all those missions as a chore, but about 10 missions in, when I started making progress into windurst's storyline, I couldn't put it down. No other mmo (and few games and even movies) gets me that interested in the characters than I had been with wotg's story.
Now I feel like I want to explore more, but Adoulin's story is incomplete and it seems like a shame to make some progress into the story, but then have to wait a few months to find out what happens.
Were the other addon stories (Crystalline Prophecy, Shantotto accession) any good?
Is there any other big questline with a good story that I'm missing?
Which xpac do you folks think had the best storyline?