That's kind of a weird question. That completely depends on the mode the song is using. If it's in regular 'ole natural Aeolian, then the major 6th is gonna sound awful. If you're in Dorian, then it's the magic note that sounds right. Everything is relative to the tonic and the mode.
...and that's why the most correct answer is the b5, because it sounds the same no matter what key or mode is in use (and yes, this is mostly a joke answer, but also kinda true)
Goo d question. I gotta go with the 9th