Kuroinu: Kedakaki Seijo wa Hakudaku ni Somaru is my top pick here. I've actually been looking for the official uncensored version of this for a long time. I know it exists, but I haven't been able to locate it, despite rumors floating around about it. I actually enjoyed the story here. While I did believe they dropped the ball a bit with the fifth and sixth installments, 1-4 were excellent and I'd go so far as to say it's my favorite hentai overall, and I'm particularly fond of the scenes with Chloe and Alicia. The scene "choreography" was outstanding, the animation was great, the girls were stunning, and I felt the story could have continued in any number of directions.
Shintaiso: Kari is another I'd love a sequel to, as gymnastics is near and dear to my heart, being a former gymnast myself, and I simply love the art, the animation, and the dynamic between the characters.
The Mahou Shoujo series, particularly Ai and Elena/Erena (looking for an uncensored version of the latter as well, and episode 5 of Ai) as I believe the story had endless potential to continue to diversify and escalate the threats, the overall story was one of the more compelling I've seen in hentai, the art and the tentacle scenes were top-notch, and I absolutely love both Ai and Elena as characters.
