I'll agree that Big Guard is the best spell in the game, with White Wind a distant second.
Every time I played on my PS1 version, a different boss was hardest. But then, back in those days I didn't have a memory card, so I had to work super fucking hard to get through things. I still remember every boss that ever killed me - Reno (first fight), Hundred/Heli Gunner (Shinra Tower), Midgar Zololm, Gi Nattak, and Materia Keeper. And I had to restart from the beginning every time.
I finally got a memory card after I made it to Disc 2
But typically, fucking Demon's Gate. Thing has stupidly high defense ratings and it deals a bit more damage than most bosses at that point. Blah. It's not hard, per se, just really annoying.
Actual hardest boss imo? Iunno. Hojo, maybe?