Getting the aeon Anima in Final Fantasy X is, without a doubt, my most memorable sidequest. You have to use the destruction spheres to get the special items in each of the temples in order to be to proceed into the temple where Anima is, and if you miss the item in the temple in Bevelle, you can't go back for it, so you're completely locked out. I only got that item by luck, so I felt really proud of myself the first time I completed this quest. The entrance to the temple is also in a place you visit early on in the game, so you get a rematch with a monster that nearly swallows you the first time you encounter it, which is immensely satisfying.
Also, if you know the game, you know that Anima is the aeon that Maester Seymour uses, and if you go to get it yourself, you get a little extra background on him, which is a nice touch. It doesn't amount to much, but between that and your success being dependent on your diligence in earlier temples, this quest definitely felt really interconnected with the main quest, a feature I think a lot of sidequests lack.
Also, if you know the game, you know that Anima is the aeon that Maester Seymour uses, and if you go to get it yourself, you get a little extra background on him, which is a nice touch. It doesn't amount to much, but between that and your success being dependent on your diligence in earlier temples, this quest definitely felt really interconnected with the main quest, a feature I think a lot of sidequests lack.