The Zhentarim, all the way (with the caveat that I'm not familiar with all of the listed organizations.)
Of course, I'm saying that as a DM. The Zhents are the "best" evil because they're neutral evil in pursuit of profit. They can have ANY motive as long as they make money off it. This makes planning plot-quests with Zhents involved really easy, but easily diabolical too. "What's that you say, the Zhents have found a profitable market selling the still-beating hearts of elven children to the drow? Something must be done!!" Man, that never get's old.
As for the Thayvians, they were "Evil", with the capital 'E' and everything, and then they decided to be the Wal*Mart of magical items. What the hell? Sure, it makes sense from a business point-of-view, but as a DM, it shoots their "evil factor" down like 10 notches. Suddently PC's are like "Well, we could fight them, but they have the best selection of wands in Faerun, and I don't want to lose access to that." Erg. Sure, it's not like I couldn't change that if I wanted to, but like George Lucas digitally inserting Christian Haydenson to the end of Return of the Jedi, the knowledge alone has ruined the original for me.
The Emerald Claw does look tres cool, but I don't have enought experience with them "in game" to realy "feel out" how evil they are. Time will tell.