Well lets review them.
Andariel: Maybe she could have been something better but really she was small time. At best she had corrupted the Rogues on her own before her career got torn down. Did she really take part in bringing down the Prime evils? A bit mixed up in her ambitions if you ask me.
Duriel: Hmm some blob of flesh who shows up with no introduction in place of what you thought would be one of the prime evils. There is a word for that kind, Tool.
Belial: Fool. I think Mo drove that point home already.
Azmodan: If I can give him any kind of credit it would be that the death of Belial stepped up his timetable way above his comfort zone. So he had to begin his attack early. Where indeed were the other five sins? My guess is they were still on leave and could not make it back in time to join the battle. So with his command structure pretty much absent he threw everything out at once. As you kill off his minions and destroy his lovely toys you can taste the panic rising in his taunts. When it finally comes down to him he has lost all control, and being unaccustomed to getting his claws dirty, fails outright.
Mephisto: Some good points have been made about him already so Ill go into what does not work for him. Of the three primes, Meph is the sniveling, anemic, weasely one. What went on after Diablo entered into hell? Notice that Mephisto was the only one of the three left when you arrive on the scene. Im thinking Baal heard the compelling stone being smashed and then said, well bro gotta get back to my business, been imprisoned for a long time and all so theres a lot of work to do. So just chill out here and ill catch ya later. Bye! And really, Meph does drop like a sack of potatoes.
Diablo: Hes a tough customer. Very powerful and is always seeking more power. However he is over arrogant, has poor leadership skills, and not very good at dealing with new situations. He does have a handful of minions but he just throws each one out with a threat ringing in their ears and leaves them. He also has a fixation with fallen angels, in particular, Izual. Honestly why did that guy come back? I thought when you killed him the first time and his spirit appeared in angelic form and vanished that it was done and signed for. Diablo must have really missed him to bring him back if you know what I mean. Finally Diablo only learned one new trick after absorbing all the other souls of the lords of hell. That was to port you into another realm where he can summon shadow versions of yourself. Otherwise he sticks to his old game plan, fire, lightning, cages. Rinse and repeat. Finally he always seems very frustrated that he keeps getting beaten. You would think he had learned a bit more about the abilities of people after having died at their hands multiple times.
Baal: Yes I have saved the one I am leaning towards for last and Ill tell you why. Terror, hate, anguish, pain, lies, and sin are all personal, abstract, mental and emotional constructs requiring an outside effect to exist. Most of these require sentient thought. All can be ignored, or at the least lessened by the mindset of the one who feels them. Destruction however is a force. All which is created will ultimately be destroyed, living or not. Baal is that. Where was he when his brothers were slain? Obviously lying low and giggling his head off as he watched it happen. Baal needs no motive; he has no long term goals. Yes he seeks power but for why? So he can destroy even more. He does not need to plan, only to wreak mindless havoc wherever he goes. He also is very good about how he uses his tools, all of which are intended to be destroyed as soon as he no longer needs them. Duriel and Nilithak, they served their purpose to perfection. Hes the kind of guy who laughs his ass off as wave upon wave of minions die in front of him. Hes a bona fide maniac and he loves every moment of it.
Baal he's the freak to fear!
