What are the Key things that Must be done for 100%? (Help!)

So far I have played through 3 times| 86% 1st go 99% 2nd and in my third now at 99%.

Things i missed in first 2 plays:
-Never calibrated all the towers (couldn't be bothered too annoying)
-Only got the Choco Eater as the culprit in the Mi'ihen Machina Mystery
-Never completed the "Here comes the bride"* or Publicity Campaign side quests.
-Never raised any Chocobos above level 2 or found any items or dungeons.
-Did not complete the puzzle cave in Thunder Plains at Chapter 5.
-Lotsa other stuff >.>

So what am I missing?

Well, seems to me a lot of people forget to check out the Moggle suit at the very beginning of the game...

did that :(

The most annoying part is watching all the video spheres. I didn't like it much anyway, and there's some of them that keep going for a long time after you think they're already over.

^^ yeah the commspheres suck. and the chocobos and side quests suck too. i gotta admit i havent completed the publisity campaine either, i tried really hard once but i didnt get it... but i did get the bride one.

Buy the guide.
It's better than just asking us because there are about 100 little things that you must do.
And they have the completion played out in every chaper in the guide.
If you are behind, start it over.

A lot of the mini-games do not contribute to your % total. You just get nifty stuff mostly.

Unfortunately the official guide isn't entirely accurate. The best I could get was 99% in a single playthrough.

I've heard that the easiest way to get that extra % in the next playthrough is to give the Awesome Sphere to New Yevon instead of the Youth League. I've also heard it's possible to get 100% in one playthrough, but only if the Youth League gets the sphere.

To get at least one of every item, object and GG, you have to do three playthroughs. Including ones where you deliberately fail certain missions.

Here's something that it seems everybody's been overlooking (so far in this thread), you have to watch every story sequence, and you have to listen to Maechen when you reach the Youth League HQ at the beginning but you CANNOT interrupt him even if the game gives you the option to. When the selection comes up, do nothing until he continues his story (can't remember 100% if it actually just continues if you leave it be, but it does sound accurate). His story can be painfully long, but you must endure if you want that 100%. And what I meant by watching every story sequence, is that you must not press any buttons to "speed up" that part of the story/game, including every (dress)sphere you get. Just sit back, relax, and enjoy the FMV, or the narration or dialogue or whatever that occurs on the screen. If you were able to get 99% on your second play-through, those little nit-picky things may be the reason why you didn't score 100%.

You can find a couple of story completion guides at the list of Final Fantasy X-2 guides at GameFAQs. I recommend following the Optimal 100% Story Completion Guide because it uses the North American localization as a base for its walkthrough parts. I, myself, have gotten a 100% playthrough only because I followed a walkthrough by the letter.

It is important to note that the game contains more than enough storyline points for you to increase the percentage rate of the game. Off the top of my head, I seem to recall the maximum percentage available in the game being around 103%.

The biggest problem I had was in the first chapter. You cannot walk into the Moonflow from Djose highroad, you must talk to Tobli by landing in the Moonflow from the Celsius first to get full completion. Drove me nuts for a long time before I figured that one out. No walkthrough I've seen mentions it.