Its difficult to say, which is most important, because frankly they are all important. But if I had to make a priority, then I think, its important to begin with the most fundamental things, that are also easy to fix, so my suggestion would be:
1) Bankroll management / game selection
Without this things will always go wrong, no matter how good you get, and its not a topic, that require hours of study or practice, so its a really easy fix. You dont need to have the "perfect" plan for bankroll management, but having a plan and following it is crucial.
2) Preflop ranges
Every hand starts with preflop, so getting this at least somewhat right prevents problems on the later streets. You dont need to perfectly memorize ranges for all situations but at least stop open limping or playing way to many hands and adapt a TAG strategy, where you always consider raising before limping behind and 3-betting before cold calling. There are spots to limp behind or cold call, but the natural tendency of a beginner will be to do this way to often.
3) Postflop play
This is, where the real game is played and something, you will never stop improving. The first priority should be to learn a simple TAG strategy, where you bet or raise more, than you call. The CC 30-day course has many sections about different aspects of postflop play.
4) Mental game
I kind of wanted to put this as number 2), but its not something, that can be quickly "learned", and for that reason it went down to number 4. Most people including myself still experience tilt from time to time even after many years of playing. So its something, you just need to learn how to handle and gradually get better at. Its related to bankroll management as well, since many people move up "to win it back quickly", when they go on tilt. Having (and following) a bankroll management plan at least protects you from this often bankroll busting mistake.
5) Exploitative strategies
Once you have your baseline strategies in place, the next step is to identify ways to exploit, what other players are doing wrong. Like value betting thinner or bigger against fish, not paying off nits etc. Also identifying situations, where the population deviate from GTO. One example could be overfolding to 3-bets from BB, because most players will tend to mainly call with their marginal hands closing the action.
6) GTO concepts
This is actually related to 5), since you need to know, what you are adjusting from. And going back to the example with 3-bets from BB a solver can tell you, what the opponent is "supposed" to 3-bet. And then you can use that to determine, if you think, this is actually, what your player pool is doing on average. Its also kind of related to point 2 and 3 about getting your "baseline" set, because a baseline is the old school way of approaching a GTO strategy, where you try to avoid making any big mistakes.
I put this last, because its very advanced, and I dont think, a new player will benefit from burrying themselfes in solver analysis. Does not mean, it should be completely ignored, but you can get far in poker without owning a solver, and solvers use a lot of partials, which is not very practical to implement for a human. Like maybe you are supposed to check a certain hand 28% of the time and then also use 3 different bet sizes. And this can easily be simplified to just betting 100% of the time with one size without leaving to much value on the table.
but in future maybe i will, i just watched some finat tables reviews or stream and just watch the game of player that i watch on stream. Good luck in your game!
