from the Diablo booklet the story said that Leoric was at first righteous and with his actions he helped the people of Khanduras that accepted him as their ruler. Diablo chose him (after he appeared to Archbishop Lazarus' dream and forced him to free him) but still even though he made him mad he wasnt able to fully control him.
All of the Three except Baal (who was imprisoned and chained inside Tal'Rashas body) freed themselves because of the information Izual gave them about the Soulstones. That doesnt show much of their strength.
just the fact that Baal survived the longest and caused the greatest devastation (counting in the Worldstone which could mean the start of Diablo 3) and mayhem, and of course the most victims (both during the Sin War and Diablo 2 he was the last one to go down) he should be one step higher than the others.
Mephisto was the oldest brother of the Three, that explains why he appears to be running the show and he definitely is an extremely strong evil entity (he destroyed Inarius' army and corrupted the Zakarum council by himself) but I still dont think he'd accomplish as much by himself.
Diablo managed to free himself and the other two from the Soulstones and thats his biggest plus so far
All of the Three except Baal (who was imprisoned and chained inside Tal'Rashas body) freed themselves because of the information Izual gave them about the Soulstones. That doesnt show much of their strength.
Mephisto was the oldest brother of the Three, that explains why he appears to be running the show and he definitely is an extremely strong evil entity (he destroyed Inarius' army and corrupted the Zakarum council by himself) but I still dont think he'd accomplish as much by himself.
Diablo managed to free himself and the other two from the Soulstones and thats his biggest plus so far