ahwawawa... this is rough. I can eliminate some choices right out, still haven't played My Girlfriend is the president, and I couldn't get into either Saya or Hoshizora. Clannad, I did enjoy, but it had some really bad moments, and I don't think it will stack up against the other powerhouses in this poll. Uta was fun, and I quite enjoyed it, but the end was pretty horrible.
So I think in the end, my problem comes down to three VNs I love to death. F/SN, Katawa, and MLA.
Fate was one of the most epic things I've ever experienced. And I love Rin to pieces. However, the adult content felt