List from 2chan, as reported from kotaku if anyone is interested :)
http://kotaku.com/5134147/a-list-of-the-best-erotic-games-of-2008
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The top two games look to be music games(not to mention concerto note at the 4 spot), so that may say something about what those 2channers want from a game. To help you with your fapping browsing, I have included some of the links to the cgs of Sumaga, which shows why it is number 1. Of course, some may say that the story/game itself could be the reason, but any eroge with a twin tailed loli(who suspiciously looks like sakura from Da Capo) gets a vote from me.

Yandere Simulator “Could Be Renamed”
Studio FOW Short Totally Tentacular
Phantom World Stretching Expectations
Charming Futaba Nendoroid
How You Wish KyoAni Made Phantom World…