I don't recall hard bosses in FFVI, maybe Ultros (damn tentacles). The rest is a pushover, including Kefka, which I beat at around level 35 (highest level).

FFVII have that Jenova thing at the end of Disc one. That one was tough, but I admit, I wan't using my brain then. A year later, I checked and wow! A water Ring! I wonder if it works, and it did.

FFIV had some hard bosses. The one that I remembered clearly is the delta twins or something. Yang's kick just !@#$

FFVIII, with the junction thing, was simple really. =/

FFIX don't have any hard ones, unless someone would try to point out one. Maybe the dragon in the bermucia gate (can't remember the name)? *thinks* Yeah, that was tough =/

If you count FFTactics, then I would go for the Execution site.