Major Pentatonic
The absolute fundamental skill for learning melodies quickly, Improvising with ease, and outlining harmony. Mastering the pentatonic scale in every key is essential for any mandolin player.
Major Blues
What sort of practice will give you the biggest return on your time?
Breaks, melodies, and backup are most often composed of this lesser known blues scale. It’s just one note away from the Pentatonic Scale, but that one extra note adds a ton of color.
That one extra note is the minor third, which nods at a darker, more colorful sound than just pentatonic.
In the key of D, that makes this scale D, E, F, F#, A, and B
Slide up from the minor third to a major third to achieve a brighter sound, or down from the minor third to the second for a darker sound.
This scale alone works alright over all of the chords, but it sounds even better when you switch scales based on the chord in that moment: IE: G major blues over G and A Major Blues over A.
You can hear renowned guitar players Bryan Sutton and Marcel Ardans talk extensively about this scale in this interview:
Bryan Sutton Teaches Advanced Improv Exercises For Bluegrass Guitar
Working on this scale in all the bluegrass keys should be a foundational exercise for any mandolin player.