Hmm... tough. Excluding the Water Temple, I honestly love all the adult dungeons. But if I had to pick one, I'd go with the Fire Temple. Really like the setting, mood, music (all verisons) and situation with freeing the Gorons (even though it really makes no sense when you think about it... have they really been imprisoned for 7 years, or were they just recently imprisoned?!)
However, there's no competition for my least favorite: Jabu-Jabu's Belly. Never did care for it. In fact, I'm playing through OoT 3D right now, and I had to force myself through it.