Gonna have to go:
CoP > RotZ > ToAU = WotG
I felt like i was just trying to get through ToAU for the sake of completion. Granted, there was/is tons of content and it introduced some amazing aspects to the game, but i always felt like i was just there because it was there. ToAU was like the "new phase" of FFXI. New players could jump almost straight into it. It became like a new standard. A lot of "old guard" players i know never really developed any kind of attachment to ToAU. It was there. It was new. Get it done. Get new stuff.
WotG isn't complete yet, so i can't really compare. I like WotG so far, but in an unfinished state it can't really compete with the other expansions. I fear that a lot of the systems being introduced are moving FFXI too far away from it's origins and i'm not sure how i feel about that.
RotZ is how i grew up in the game. Linkshell events for Missions, Sky access, beating Divine Might, farming Sky Guardians, first Kirin fights, infinite chain d. weaps camps, fighting Dragon's Aery away from the JP players, Ridills, Adaberks, forming Dynamis strategies, etc etc. RotZ is all nostalgia factor for me. Parts of the game i loved at the time, now almost never go back to, but can still look back fondly upon.
CoP is a totally different story for me and a huge part of it is the music. I can hear any of the other expansions music and be like "Sweet, FFXI music." but when i hear music from CoP it just pulls me into a different place (i can still see the CoP trailer in my mind, with "Depths of the Soul" playing in the background, and remember how excited i was.). The CoP missions grabbed you in and never let go. Before they got nerfed for difficulty, they were brutally uncompromising and it really challenged how aspects of the game were approached. Suddenly item use became a huge deal. New strategies for level capped missions had to be developed. We were given incredible zones like Pso'Xja, Tavnazian Safehold, Riverne Sites, and my favorite zone (and music) in the game, Promyvion. Boss battles were huge, epic, events, Grand Wyrms, Diabolis, Bahamut, U/O, Tenzen, Promathia, and so on. CoP took the mythology were were all familiar with from RotZ and crafted a story that was complex and satisfying, filled with some of the best NPC characters to date. You wanted to finish it because you wanted to know what happened, not just because it was there.
Despite being "small" in comparison to something like ToAU, CoP changed a lot about how we played FFXI.
XI Wiki



