Interesting question...
I've already found the perfect Hentai-Game, so maybe I should list some of the things; which I like about it.
I'm talking about "Sexy Beach 3". I haven't played many hentai games so far in my life, but I think it will be very hard to find a better one than SB3.
I like about the game that it's endless. Good, there is the point, where you had got every cloth, hair style, etc., but even than it's a good variety to pick a girl, dress it up and than fuck her and not just sit and watch.
What leads me to my second point: The game is very interactive. You don't just klick through pictures and make sometimes a decision, you are acting.
Then there are the things around the sex: The sometimes quite funny conversations with the girls, the dressing of the girls, the nice backgrounds, etc. It's not just about fucking girls, it has substance.
And propably the best thing about Sexy Beach 3 is the possibility in it to be creative. I am not an artist, but I love to dress the girls in that game. It's kind of a pervert art xD
So, all in all: A good hentai game should be about more than fucking girls (but of course that should be the main focus ^^) and should be very interactive.
The best game would be Sexy Beach 4, but I think this game will never be released...