I found Seifer on Disc 3 to be the easiest. But that's because I had all my GFs, and all junctions with strength maxxed out. I actually had to be careful not to kill him too quick, as I was trying to draw magic from him.

I actually found Ifrit to be harder, because I was at low levels, and had no extra abilities learned.

It all depends on how your junctioning is, and what GF abilities you have when fighting any boss.