Over a chord, you improvise from a scale that contains it. Which one depends on the chord's quality and role: Dorian over a ii chord, Mixolydian (or the altered scale, or Lydian dominant) over a V7, Ionian or Lydian over I. The two notes that matter most are the guide tones — the 3rd and 7th — because they define the quality and thread the progression together, the 7th of one chord stepping down to the 3rd of the next. Pick a key to see the ii–V–I with its scales and guide-tone lines, or type any chord.
Guide-tone lines (3rds & 7ths), voiced for smooth motion: