Well the series isn't over yet, so people still have time to perform. Some people are waiting for late entries from Forsaken such as Demandred, Moghedien and Cyndane.

Disclaimer : All the Forsaken have been utter failures, but let's see if Mesaana is the smallest failure among them.

The first reason that Mesaana may be the greatest is that she's alive. There are very few of the original Forsaken who have survived Rand and his allies. Most of the most powerful Forsaken are dead, but she has managed to evade him. Not only that, but Mesaana may be one of the few Forsaken who flies below the radar of both the Shadow and the Light. Unless I'm mistaken, Rand has never mentioned her or thought of going for her, though he frequently mentions people like Graendal or Demandred.

Mesaana did quite a good job in the Tower up to book 12. It is true that she walked into a well established infrastructure of darkfriends, but she certainly played an active role in steering Alviarin and causing havoc in the Tower. She sowed chaos as well as any of the other Forsaken. Of course, Egwene undermined her utterly in the last book and brought all her plans to nought. Mesaana was convinced of a victory in the prologue of that very book, showing how severely she underestimated Egwene.

Mesaana also managed to ally herself with two of the most formidible Forsaken - Semirhage and Demandred. It's always remained curious to me as to why they allied with someone as mediocre as her, but it seems that she has subtle academic talents that even they can't ignore.

Mesaana orchestrated Rand's capture in LoC, which was ultimately undermined when he was rescued by Taim. Unfortunate, but if one thinks about it, an enormous victory for the Shadow if one considers their other catastrophes.

She was humbled by Moridin and SH for not showing up at the cleansing, not for her incompetence, but for her arrogance and indifference. This was a mistake on her part, but an example of her defying the Nae'blis more than being useless.

If one lists the scanty microscopic victories attained by the Shadow, it seems that Mesaana has kept to together better than most. She's even achieved a few things, and live to tell the story.