I've been there! Climbed all the way to the top benches and sat there for a while. The acoustics are truly impressive. It's not at all loud and echo-y as I expected, but perfectly clear.
Ancient Theatre of Epidaurus, Peloponnese - Greece :
It is regarded as the best preserved ancient theatre in Greece in terms of its perfect acoustics and fine structure. It was constructed in late 4th Century BC and it was finalized in two stages. Originally theatre had 34 rows of seats divided into 34 blocks by stairs and walkways. Due to its incomparable acoustics, the actors can be perfectly heard by all 15,000 spectators, as you can even hear the sound of a pin dropping.
