I actually find all of them interesting. I am amazed how many questions people have about very common phenomena involving the Sun, Moon and Earth, Seasons and Eclipses. I am also surprised how few questions people have about the stars and the Milky Way.
I enjoy questions about cosmology and black holes, especially about the fundamental properties and issues behind the theories. I do not enjoy answering questions about what it is like falling into a black hole, or whether we can travel faster than the speed of light. I am surprised there are not more questions about the conditions near the Big Bang and what might have happened 'before' this event. I am thankful there are not many questions about string theory because the general public should not be asking such things! Not even professional astronomers understand the theory at a technical level!
The most interesting question I recall answering is how gravitons can get out from inside a black hole to produce the gravitational field we see outside the event horizon. My least favorite question was whether anyone has had sex in space yet.