Best weapons can be found by doing "Jobs" in the Guild. They come with no additions so they can be upgraded to suit your play style. The Jobs are found in post game and they are the highest level jobs.
I think the Ultima weapons that can be obtained from the menu's "Record" section for learning all weapon skills are better, since they halve BG consumption for skill usage. Even the in-game shop exclusive weapons are so dull that I stuck with the Ultima weapons until the end.
