Frankston South is the best bit, the nearer you get to Mount Eliza (border with Humphries road) the better. Frankston heights is OK, the area between Cranbourne Road and the High School is OK, the area around Kars Street is OK. Frankston North is probably best avoided, Karingal is probably too, although it is coming up. Centre of Frankston is mainly commercial, not many people live there.
HTH
BB