This is just my opinion but the Shukaku is the hardest TB to control. If Gaara gets upset he doesn't take on a cloak form. It's just full on Shukaku so there's no time for anyone to get him under control.
When Gaara fought Naruto, Gaara couldn't control himself. All these flashbacks kept coming from his childhood and it was the Shukaku of the Sand that was controlling Gaara. Gaara was fighting hard to not let it take over but in the end that's just what happened.
So therefore, the Shukaku is the hardest to get under control by will power alone. Unless your Killer Bee that is :D