Where to capo for Ab Dorian
Put the capo at fret 6 and play Ab Dorian using D Dorian shapes. That is the easiest of the eight positions ranked below, and it is what most players reach for. Against playing Ab Dorian open, it drops the mean difficulty of the key's seven chords from 5.6 to 3.4 and gains 16 ringing open strings.
You want Ab. Capo 6 and play in D.
All eight positions, ranked
Each row transposes Ab Dorian down by that many frets to find the shapes your hand would actually make, then scores the best voicing of all seven chords — fewer fingers, less stretch, closer to the nut, fewer muted strings. Lower is easier. Standard tuning (E2 A2 D3 G3 B3 E4).
What you are playing
The seven chords of Ab Dorian are Abm, Bbm, Cb, Db, Ebm, Fdim, Gb — the ones the ranking above is scored against. Their diagrams are on the Ab Dorian key page.