Art, story, characters in that order. I think those are the most valuable parts to any good porn game. I can take or leave animations since they sometimes cut into the quality of the rest of the game for the sake of making animations. Customization does not matter at all to me as long as the listed 3 are quality.
Its all about the story.
Love of Magic has a great story and I bought it off of steam and now have well over 1100 hours of play in all 3 books.
Sisterly Lust 200 hours of play and it also has a good story, bought it on Steam.
2-3 free ones on Steam.