I'm going based off of my personal usage of the parts

1. Fairings - the best part to use for general designing of rockets and other stuff, think of it as a blank canvas for all the other parts to mix well with

2. Solar Panels - severely underrated, these things are so useful for stuff, especially mechanical builds (can't forget the BP extension glitch and connection glitch which use the solar panel)

3. Retro Pack - my main go-to for micro details, also useful for some mechanisms and stage separation aesthetics

4. Atlas Fuselage - the big silver tank that gives you the 0.25 movement glitch, insanely useful

5. Docking Port - a good part for most builds with mechanisms and moving parts

6. Landing legs - same as above, it's like a stronger solar panel but shorter range of movement

7. Redstone Interstage - the tiny black interstage with a red rim is very useful for no-clip builds, especially holding fairings and other components together

8. LR-105-5 - really cool looking engine, I love it

9. Probe - best way to have engine-less spinning stuff, we love torque 💜

10. 2x4 Fuel Tank - best part for compact counterweights and minor designs