For me, it's a tie. The well scared the crap out of me for my first few play-throughs, and the ice cavern bored me to no end in the same runs. The awkward traction the ice cavern gave me a challenge, and I love challenges, but the fact that you never know what's coming in the well really puts them in a tie. Love them both because they give you a break from the key hunting and just focus on puzzles and fighting.