Here is the hole in your theory: any attack or fight with the One Power is potentially an instantly brutal or fatal force. So she wouldn't have to predict the exact nature of the attack - does it matter if you are hit with balefire, a needle-thin dagger of Air or a massive lightning strike? On a metaphysical level, yes, but in terms of your present circumstances, death is death. I just don't see Graendal pulling a Lanfear/Ishamael and standing around taunting an opponent who has access to the Choedan Kal when she can just run away. She's just not that kind of villain. I think she's the most dangerous because she'll put aside her pride to run when necessary, but is not afraid to fight in the right circumstance...and to me, when the Dragon Reborn shows up on your doorstep with the same figurine he used at the Cleansing, it just doesn't seem prudent to try Compulsion. When she was attacked at the Cleansing, she didn't count on Compulsion to fight, she may have used it once, but almost immediately she switched to killing weaves.

That being said, I think the smartest Forsaken in terms of the marriage between survavibility, intelligence and Talents/strength in the Power outside of Graendal are long dead - I basically think they were destroyed by the bullies or the most selfish/aggressive Forsaken during the War of Power and The Breaking. I'm sorry Forsaken fans, I really do think we've seen the relative bottom of the barrel - what was even funnier was when Graendal called Mesaana a "second rate Chosen".

Demandred may be smart, but his ego makes him inferior as an opponent. He won't fight to win; he'll fight to win as to win the most technical points possible. Even worse, he only wants to kill the Dragon, which makes his weakness very apparent. The only advantages he has are superior channeling strength and a brilliant strategic mind, neither of which gaurantees victory with such character flaws.

Lanfear and Ishamael practically painted bullseyes on their foreheads, counting on bullying Rand into joining them. To make their intelligence/wisdom levels even worse, Lanfear KNEW Rand couldn't be bullied and had the added evidence of Rand killing Ishamael, and still proceeded doing the same thing. She was actually the closest to turning Rand, and she wasted her opportunity to Compel him to her side or outright capture and train him with nagging and stamping her foot. Both are very powerful and very stupid. I can't even list how dumb Ishamael was - he brought a quarterstaff into a fight with a "3rd Age" barbarian, when usually it's the modern people who back off from more "barbaric (ie: violent and weapon smart)" opponents.

Semirhage couldn't keep her eyes on the prize long enough to collar Rand and Travel to the Bore and Sammael fell into a partnership he lacked the cunning to manipulate (he also ended up stamping his feet when he realized he was set up as a target, but he didn't change his tactics). Aginor has potential to be at least the most "book smart" of the group, but lacks the facilities to take advantage of his capabilities and Moghedien doesn't have the strength, courage or timing to rely on her Talent or do anything but get pulled down into her own plans. Ladies and gentlemen, let's remember when she had Elayne and Nynaeve in her hands and instead of Compelling them into her servants, she left them to do their own thing.

At least Mesaana had almost captured him, though if I were in her shoes, I would have Travelled to the Aes Sedai camp, taken the box he was in along with enough Black Ajah to maintain the Shield, and went straight to the Bore.

Graendal is the most lethal because IMHO she doesn't get pinned down with one strategy that easily. If she sees you coming for her, she goes to the other end of the world. If you know what she's doing, she'll run an opposite game just to throw off your scent. I really liked how circumspect she could be in one moment and the next "Oh, let me pop over to Shara and take their rulers, and be back in time for sorbet". Very effective and no hand-wringing (like Moggy), philosophizing (Ishy), gloating (Lanfear/Sammael/Rahvin) or engaging in personal hobbies (Semi and Messy).