Demandred was only a little behind LTT in intelligence. He gave LTT an IQ of 170 and Demandred 160. I should find the quote before I misquote him, though.

I think that Ishamael, Semirhage, Graendal and Aginor had similar IQ's though.

IQ doesn't make a person competent though. I think Aginor may have been the greatest mind among the Forsaken, but he was probably the least competent socially, and as a Forsaken his only real place was in a lab trying to make new monsters.

Demandred has huge personalities flaws which undermine his greatness. This is true of all the Forsaken. However, he is obsessed with the idea that he is the greatest man alive. Unfortunately he does almost nothing to prove it. He's riding on a wave he started in the Age of Legends, which is getting very small I'm afraid. Maybe Rand isn't a super genius, but he has the skill and temperament to kill any of the Chosen, and the logic and mental aptitude to find ways to perform great feats like the cleansing of saidin, and the destruction of Graendal.

The least flawed Forsaken is probably Sammael, who is similar to Demandred in most ways and turned for the same reasons. He really goes for his victims though, and was proactive as a Forsaken on many levels. There is almost nothing he didn't try against Rand - directly attacking personally or via his soldiers, defending Ilian, plotting with other Forsaken, using darkfriends and even a truce at one stage. He was very unlucky to die - thwarted by Moridin and mashadar.