The "Confrontation". The way the actor manages to convincingly play both Jekyll and Hyde, to the point where you can almost forget that it's only one actor on stage instead of two, is phenomenal. It's probably the best way to verbally describe people suffering from a Split Personality.
Many of Jekyll's songs. The role is sort of a meta-CMOA because it's so damned difficult.
The original ending to Act 1 - as Hyde's singing the reprise to "Alive!" he sets the bishop he just killed on fire. It's terrifying. And epic.
A version of "Alive!" while the show was still in its touring, pre-Bway phase that had *a fight scene*.