I'm thinking Hiei by a small margin. Close one though. I think the biggest difference is the DOTDF. In their stats it's arguable who's superior as they both have highly impressive strength, speed, durability, stamina and swordsmanship skill.
True, but I don't think he has what it takes to put Hiei down realistically. He could block Bui's axe bare handed before absorbing the dragon. After absorbing it he could tank Bui's attacks effortlessly. Also DOTDF is probably a bit worse than the Kishin Lasers. Even in its initial use, it incinerated a FIRE demon to a black spot on the wall instantly.