Personally, I think all of them is worth it (eventhough my first post saying Lulu's not worth it). They power-up Aeons which is useful. Even

Lulu and Kimahri weapon is useful and doing the sigil is woth it.
Their attack really become strong and they break the damage limit. They power-up Ixion and Shiva. I did 99999 damage with Lulu's Ultima and Kimahri physical attack hit about 40000 damage now.

All of the weapon is worth it.