Hard Mode Boss Difficulty Ranking Easiest - Hardest:

Roche - Easiest human boss since almost all his attacks can be countered.
Specimen H0512
The Valkyrie
Swordipede
Scorpion Sentinel
Crab Warden
The Arsenal
Reno
Airbuster
Failed Experiments
Jenova Dreamweaver
Rufus Shinra & Darkstar
Abzu second fight
Reno & Rude
Whisper Harbinger
Sephiroth
Hell House
Eligor