I think that the best *overall* incest game is Dreaming of Dana simply due to the story in and of itself. It actually FEELS like a story, there's feelings and sexual tension hidden under the surface, which to me is far more important than the art as you can easily play any one of 100's of different adult games for pics but very few have such a deep and coherent story that makes the characters feel (at least somewhat) real.
Summertime Saga is my 2nd choice but only due to the current lack of any depth to the story (as of yet). These 2 are easily the best out there and I, personally, prefer a good story over pics. I think Summertime Saga's setup, pics, etc are better but Dreaming of Dana wins my vote in this simply due to the presence of the story along with the actual struggles and doubt that comes with an incest relationship. It has a degree of realism that makes it an incredibly good story where (as of yet) Summertime Saga does not. If Summertime Saga had more story than it currently does, then it would have won my vote (but I love both games).
Tied for 3rd, and 4th places are Dreams of Desire (amazing renders), Dating my Daughter (again, story hints at realistic situations and is well written... if only the dev would stop dragging his dick in the dirt, maybe we could finally get rid of our blueballs -_-). Teacher's Pet doesn't have enough content or context for me to vote for it one way or another (though I LOVE it's animations and art-style and look forward with great anticipation for what it has in store for the future). Honorable mention to the Twist but I would never vote for it for the simple fact that it isn't actually an incest game as the MC and his sister are both adopted, otherwise it would take the place of Dating my Daughter (due to the frustratingly slow progress of DmD).