Crescendo

This is ridiculously underrated. It gets straight to the point and never lets go of its powerful grip. Every route offers something different and appealing. Kaho's route is a great way to open since it kind of sets the most balanced tone. It's a good melodrama. Kyouko is definitely my favorite girl in the game and her route is really cute and a great breather for such an otherwise dramatic game. The two heaviest routes, belonging to Yuka and Ayame, are clearly the best and anybody with a soul is sure to be moved or challenged by them. Ayame's in particular is bound to leave a sour taste in many people's mouths, which only makes the game, and its protagonist, more interesting to me. Kaori's route doesn't have quite the same power to it but I don't think it's far behind Kaho's or Kyouko's, it's very good for a teacher-student age-gap route, and could also be seen as something of a breather as its not quite as dramatic.
The bonus route with the piano girl is a lot worse, but I'm going easy on Crescendo despite that because it's just a bonus route and the game feels totally complete and fulfilling without it.
Going into Crescendo I had no idea what I was in for, but it went way over my expectations.
It's worth noting that the voice acting performances are of unbelievable quality, The writing is spot on and never drags, the Shoujo artstyle for a Bishoujo game is really unique and suits the game perfectly, and the old-time music gives it just the classical edge it needed. The writing is melodramatic, but classical and has integrity. Highly, highly recommended and exactly the sort of thing that makes Eroge not only worthwhile, but a true art.