Sweet baby Jesus, please read the last 4-5 pages of the guide thread for more NIN REMA discussion than you will ever need.
But...
Yeah if Aeonic is accessible to you that's great. It's often the best NIN weapon and never does worse than a relatively close second.
For non-aeonic, Kannagi beats Kikoku pretty easily in nearly every circumstance. However, Kikoku is cheaper and easier to make (no tedious Empy Abyssea trials) and is still a very good weapon that is a nice step up from non RMEA.
Kannagi is the best NIN weapon in most situations with low buffs (like, 1 buffer or less unless it's a really good GEO with Idris or using Bolster). Some people seem to be convinced that you should only use a melee with more buffs and making an ultimate weapon just to be best for low buff scenarios is not worth it, you'll have to decide that for yourself. PLENTY to read about that in the main guide topic.
Nagi is not competitive for DD purposes, and debatable whether it's even best to use for its best purpose of tanking. Given the big price tag, not recommended for anyone but the biggest NIN completionists.