Ruby WEAPON.

Ozma is too random to start, Aeronite and Ereshkigal can be beaten with the very easily accessible Bio and Artemis Arrows that does not unlikely deal damage on a 999999 level if you continuely use it.
Yiazmat and Omega Mark XII are very hard but they are the kind of super hard bosses that require you to have super equipment anyway to reach them. Proto Babil and Geryon are also pretty hard, Zeromus EG is easier than that. Neo Shinryu, Shinryu, Omega, Omega Mk. XII and Enuo are very hard but Final Fantasy V is pretty easy to get yourself some very useful abilities and equipment. Vercingetorix, Shao Long Gui, Yomim and those other freaks never really appeared to me like the comparable super bosses, Cie'Mar Atilla did though. However he also was just a matter of "survive until Bio has done its effect". There are others that are hard or still hard with equipment and easier with but in general: Ruby.