I feel like we've seen this thread like every month. I'm not saying that as a bad thing, it's good and normal that newer players wonder what is the most efficient way to spend their time. I just think that the answer to this question is less about what activity you should do and more about how much time can you spend on it.
Everyone above me in this thread is right : playing jump maps/DM will make you better at jumping/DM, watching yours or other people's demos will help your gamesense, scrimming and to a lesser extent (imo) PUGs will generally help with everything. I don't think there are like trade secrets like "play nosplash with double your sens" or something, if you look at every player or team that climbed quickly through the ranks, they all played a lot of the game, and I think that the time spent is more important than what you did assuming you were always trying to learn.
So I think my answer to the original question is : whatever you can do the most without burning yourself out. If that is demos, DM or even pubs (I know people who grinded pubs to train and it paid off for them). Sure, in a vacuum it's always going to be more efficient to ,for example, play a jump map rather than a pug if you wanna get better at jumping, but if you hate jump maps you won't get much out of forcing yourself to play them.
I read somewhere than when you ask people for helpful advice, they tend to answer with things they wish they knew back in the day and this is the case for me right now, so sorry about the rant and hope it helps the question a little bit.