Personally, there are a lot of completed videogames that deserved my time, but so far (Hentai only speaking. Otherwise my favorite complete games are other of course):
1) Artificial Academy 2, without a doubt. Illusion games aren't top quality games, and we already know that. Everyone does. But after this one? Only trash. Personally speaking, only Sexy Beach Premium Resort can be saved, but other than that... Honey Select? The actual Playhome? Secrossphere? MEH.
2) Even though it's not yet completed (but technically speaking, the main game is. What is coming out are DLC's - speaking of the second at least), Custom Maid 3D. I mean.. the amount of things that you can do within this game is impressive, and things keep coming out like there's no tomorrow. Lot of personalities for the maid to choose from, the "train phase" is more fun than the one's of PlayHome for example, the fact that with an expansion you can marry your maids is top notch for me. I mean... there are a lot of things, positive things about this series.
3) Strange enough, all of the Taimanin series by Black Lilith. Even though are "cruel" - but never, NEVER, like Starless. That is seriously fucked up - and someone don't like them, I honestly enjoyed the story so far! I'm still playing the third chapter (and then Yukikaze and Murasaki), but It's really, really good for me.
4) Even though it's not completely H because has a hell of a story firstly (positively speaking of course) and the scenes are the secondary thing, all of the Grisaia series.
5) And like the abovementioned, this one, my first love and the VN that introduced me in this wonderful world: If My Heart Had Wings. Along with another production of Pulltop, Koisuru Natsu No Last Resort. These two games are wonderful, in every way possible: from the H scenes, to the background scenes, to the charaters. Everything is perfect and still today I listen with pleasure the OP of the first one and the Main Menù theme song of the second - they're relaxing as hell.
Yeah, maybe I should've said only one, but this is my top five which will not change for anything in the world.