Dad/Daughter is the classic, but I also enjoy Brother/Sister.
Whole family with everyone is fucking everyone is fun, too.
Very interesting (but also unfortunately very rare) is Sister/Sister and Mother/Sister.
What I don't like is solely Mother/Son stories and shota.
Mother-son then father-daughter
I wish there were more games where you start out in an intimate relationship instead of 90% of the game being the same corruption/seduction loop as Milfy City or DmD