(Any Key just depends on the chord you're on or approaching)
G, A, Bb, C, D, Eb, F, G, Ab, Bb, C, Db, Eb, F, G
(Similar to bigtime's post)
F, E, C, G, Bb, A, F, C or F, E, C, G, C, B, G, D (and repeat going down)
C, Db, F, F, Gb, Bb (going up or backwards going down)
(Same as the above, but notes in a different order)
Db, F, C, Gb, Bb, F
You can also play....
Db, F, C (going up/down in 4ths or 5ths, cycling through the circle of 5ths)
It works really well, when the last note of the run is the 3rd or Maj7 of the chord you play when you end.