I would most likely have to say somewhere in the area of 2-3. It was a rather easy battle (If you consider just the sword to sword combat.), and it also felt rather repetitive. I don't care if it was the final battle, it shouldn't really be that repetitive.
If you consider the other parts, I'd give it a 4-5 with how randomized Ganon was. The horse pursuit required quick reaction to avoid Ganondorf's phantom army, and that made it difficult the first two times. However, unlike most Zeldas (where no matter how often you play, it'll still be a bit difficult), this battle gets seriously easy after the 3rd re-battle.