I would go with Dark Link. Even though he is only a mini-boss, he is still definitely one of the harder bosses for me in Ocarina of Time. The only problem was that the majority of the time he was just mimicing you unless you stopped Z-targetting him, which practically just made it a swing and hope fight. It took quite a lot of fun and potential out of the battle.
For me, the battle would have been much greater and interesting if Dark Link had the ability to use some of the items that you had acquired by this point in the game, such as the Megaton Hammer, the Bow and Arrow and Hookshot. I believe that this would have caused the player to play more defensively and think of a better tactic than said swing and hope tactic, it would make Dark Link able to produce a better tactic than just mimicing the player for the majority of the time too. The majority of damage that I receive in that battle is due to me usually messing up and stabbing at him so he jumps on the sword and strikes while you can't move, which becomes quite competitive after a couple of minutes of the fight, nevermind a couple of run throughs of the game.