Someone looking at BJJ in terms of Self-defense will look at things somewhat differently than a competition-focused individual.

In My opinion for both...
Closed Guard - understanding distance
Escape / Survival
Mount / Pins

Sport / Comp Focused
Back Control
Tech Mount

What I love and works for me in my style may not be what works for you. best advice I got from multiple Black Belts when I was a Blue belt is take 2-3 months and work 1 instructional, 1 position and explore it. You may love it, you may not like it, but you need to sample all this to see what works and what doesn't right now for you.

And be prepared to revisit positions later that "didn't work for you / hated it" as a blue that now totally works for you :D

Hope this helps a little...

Great question!

Guard - Half Guard
Passing - Toreando
Transitions - Taking the Back
Takedowns - Foot Sweeps, Snap Downs, Arm Drags, Single Legs
Submissions - Chokes (Guillotines, Head and Arm, RNC)

Half guard is a guard that works well for Gi, No-Gi, MMA, and is effective even as an older practitioner. It allows you to simplify guard retention and slow down the passer. The goals and techniques in half guard are pretty straight forward as well so it's easy to wrap your head around a game plan. It is versatile. You can attack sweeps, submissions, back takes, re-guard back to closed guard, or work back to your feet. The sooner you can develop your Half Guard the better, in my opinion.

At the white and blue belt level, a lot of practitioners do not have a ton of time invested in guard retention. I found a ton of success using toreando style passing as a blue belt simply because most people lack the guard retention skills to stop it. You can be relentless and mobile because you're on your feet, attacking side to side, and it connects to our through-the-legs, and under-the-legs passing as well. I would say try to toreando first, if that is failing then go into through-the-legs style passing like a knee cut or forcing half guard.

Work on taking the back. The best finish is the RNC. When you pass the guard, they will most likely turtle to not let you score. Get good at taking the back when an opponent turtles so you can start threatening a finish.

I recommend low risk, high reward takedowns that don't require a ton of athleticism or high levels of technique to be effective. These are foot sweeps, snap downs, arm drags, and single legs. You can chain them all together. They are relatively simple to practice and become effective at. They are low risk. They are reliable.

For submissions, I recommend chokes. If you take the back, it's the RNC or bow and arrow. If you are in mount, work your head and arm choke. From bottom, work your guillotines. By having the constant threat of wrapping your opponent's neck, they will become hesitant with where they are willing to put their head. If you've ever rolled with somebody who is constantly wrapping your neck, you'll understand how much of a menace you can be. You might have to put time into this though because you don't want to get into a habit of giving up good position diving for guillotines that aren't there. Find the balance so that you can properly punish opponents who leave their neck out while maintaining the old adage, position before submission. It's better to work towards mount and get a head and arm choke than to give up good position for a sloppy guillotine attempt.

Hope that helps!

Awesome question...

I'm primarily focused on self-defense so that definitely affects how I approach this kind of prioritizing process. Really hard to choose five but off the top of my head and in no particular order:

Knee Bump Takedown or Ko-Soto-Gake from having wrapped the opponent's arms while defending punches, shoves etc.

Escapes from Mount and Kesa Gatame - These are important because they're what I call "intuitive" positions. In other words, you see them in fights with and without training. Mount+choke or punch is a very common intuitive attack, and while someone might have no idea what kesa gatame is, headlocks are a very common uneducated standing attack, and headlock + commitment - balance = falling over into kesa gatame.

Closed Guard Defending Against Strikes - working out to foot on hip to make space to stand

Single Leg Takedown - In my opinion the most important competition takedown since it is extremely versatile, lots of ways in and lots of ways to finish, relatively low cost of failure and it is also by far the most statistically likely takedown you'll see in competition so very important to train it hard so that everyone gets their reps in defending them.

Hook Sweep - My favourite open guard sweep as it is one of the few that works amazingly well in any setting - gi, no-gi, MMA and self-defense

Bow and Arrow Choke - I love starting students on this one early - not just because it's a very high percentage choke but also because it has amazing downstream benefits when someone is working hard for it because it will naturally familiarize them with transitioning between back, mount, technical mount and sometimes even knee on belly without overthinking it.

When I used to teach, I used to emphasise having 2+ options from the same spot depending on their reaction. e.g. single leg into double leg, knee tap into uchi mata then ankle pick etc.

so for takedowns I used to teach things like armdrag into either valley drop (if you get the back) or inside trip (if you don't get the back), or snapdown into double leg (if they posture) or to a spiral (if they don't posture). For guard passing, knee slice or side smash. Mount - collar choke / armbar.

This sort of doubling of attacks was really useful for me to level up my own BJJ from absolute trash to somewhat OK on a good day, and my old students found that they had higher success rates. So my advice would be, as others have posted above, finding high-percentage moves that chain together and drilling those.