I am preforming an experiment at the moment to see if base damage dealt from an enemy effects exp gain.
Probably the deathlords. They are the ones that have always given me the most trouble.
But I highly doubt the damage they do effects your exp.
You are correct, higher level opponents that do more damage do cause the Block Skill to increase faster.
Base Damage, Skill in the Weapon Type, and Perks.
For me, it seems tied between a the highest level of fus-roh-dah, ebony bow carrying Deathlords and Falmer warmongers (in armor, not mages)... maybe the the highest level dual-wielding Briarhearts as well. Highest level magicians maybe tops, but enough PC development the *toughest* magic-wielding hombres are not that tough.
I haven't fought him myself, but I've heard the Ebony Warrior is the toughest/hardest enemy to fight.