To be honest, I don't remember anything of Hiashi... In fact if you didn't mention he was the leader of the Hyuuage I wouldn't have any idea who you were talking about. So when I give you my opinions of who he can/can't beat, I'm going off of Neji's use of the Byuakugan, and I'm going to give Hiashi the benefit of the doubt and assume he is better in a fight with his eyes than Neji. Now then, the bold is what I don't think he can defeat.
Chiyo
Konan
Darui
Kitsuchi
Zabuza
Kimimaru
Mangetsu
Hidan
Mifune
Zetsu
Zabuza: While one could make the argument that the byakugan can detect Zabuza within his mist jutsu, Zabuza is still a highly skilled swordsman and is an utter killing machine. Not one to take lightly.
Kimimaru: Kimimaru's character was killed off due to plot alone (with his sickness). His bloodline limit is truly one of the more OP bloodline limits.
Mangetsu: Not much was shown of his attack power either, but just judging from the reason why he was the leader of the seven swordsman, I'd be willing to bet that he'd be a tough one to fight. But again, this one is mostly just speculation.
Hidan: What Hidan lacks in jutsu and physical strength against the byakugan, he makes up for in his curse and immortality. One strike on Hiashi, and he's done.
