For me, it has always been reading as it requires me to concentrate much more on the subject matter than while listening. I noticed that when just listening, I can get carried away by other thoughts that might not be relevant to what I'm doing or background noises and other sounds in general that tend to grab my attention.
While I'm reading though, I somehow access a superpowers that allows me to eliminate most, if not all of the unnecessary thoughts/sounds that lower my attention while listening, and concentrate on what I'm reading with my fullest capacity.