I think in many cases where the Forsaken were defeated is that they were too overconfident and too dismissive of the abilities of the main characters because they're not channelers from the AoL and much knowledge has been lost. In most cases, Rand is being underestimated, but Moghedien I think underestimates Nynaeve at times, and Lanfear certainly wasn't expecting Moiraine to tackle her into the Eelfinn doorway.

Individual cases:

Balthamel: I think maybe he was just so decrepit that the Green Man was able to overpower him. That and the scene seems to imply that it happened very quickly.

Aginor: combination of both overconfidence and incompetence. Aginor is consitantly portrayed as an arrogant, egotistical ass, but he generally only seems to be good at making Shadowspawn. Plus he got greedy and grabbed too much of the saidin. More below.

Be'lal: Probably overconfident. He was hoping for Rand to seize Callandor, and so he could take it by force. His assumption was that Rand was ignorant and thus easy to beat.

Ishamael: Possibly unlucky. His attack on Rand in the Stone seems to have been more of an act of panic, rather than his usual manipulation.

Moghedien pt 1: Overconfident. Doesn't think that Nynaeve will remember being Compelled, and doesn't realize just how cranky Nynaeve can get. Nor was she expecting to get beaned by the Domination Band.

Asmodean: Overconfident. Thought he could easily slip away to Rhuidean in the confusion at Al'cair Dal and grab the access key to the Choedan Kal.

Moghedien pt 2: Overconfident, again. This time she realizes Nynaeve is a match for her in the Power, although untrained, so she falls back on T'A'R. Smart move, and admittedly Brigitte's sudden appearance is bad luck. She wasn't expecting to get leashed with an a'dam and I think this was overconfidence in assuming that she had all the tricks up her sleeves when it came to T'A'R.

Lanfear: Partially overconfidence, as she disregarded Moiraine. In this case, though Lanfear was lashing out in a jealous rage, so she wasn't thinking clearly at all.

Rahvin: Not really sure on this one. I think maybe he wasn't expecting Rand to attack him, since his little group was trying to bait Rand into attacking Sammael instead. So maybe unlucky. I don't think he had any defenses in place, and I would say he fought fairly well under the circumstances. But Rand was pissed at thinking Morgase was dead, the whole fight with Lanfear, and then Rahvin killing off Mat and Aviendha during the initial assault. An angry Rand is very dangerous.

Sammael: Overconfident. He always acted far too sure about his abilities.

Osan'gar: The same combination of overconfidence and incompetance again. He thinks he can just waltz right up to Rand without bothering to take cover or anything. Boom.

Semirhage: Overconfident. Thought her disguise would go off without a hitch, and it might if it wasn't for Cadsuane's various ter'angreal. Though OTOH, Semmy maybe should have had spies or something to give her an idea of what Rand's inner circle was like. Once again overconfident when she tried breaking Rand, though his use of the True Power was a very unpleasant surprise for her.

Graendal: Overconfident. She assumes she's completely in control by warping everyone to her will, and she doesn't expect a direct attack from Rand given his reluctance to harm women. Certainly NOT expecting a massive assault from balefire, since only someone seriously deranged would just casually use a lot of it like that.