Okay so when I say "single most thing" I don't mean reviewing your hand histories or playing. I mean something like a book you read, a forum you read, watching poker streamers, watching poker on TV, your dad etc etc.
I'm just curious, the reason being is because when I get deep into studying poker I actually find I waste a lot of time downloading loads of books, or sometimes I even watch videos repeatedly by accident cause I forgot I already watched it ages ago.
So back to the top, what has helped your poker game the most. If there is more than one thing, just shout.