The OSTs are probably my second favorite part of Flowers. Sugina Miki's art firmly holds that top spot, however.
I need to go through and pick favorites from each season. I feel like some of them might seem a little unusual, though. Like ete, I think my favorite might be L'enfant Perdu, there's something about it that resonates with me. It's not the same kind of tearjerker kind of emotional theme that I look for in the other seasons, but I think that might be part of what makes it a little special to me.
Thorn in the Heart from Printemps is one of those that cuts deep, and I like that in a theme. Especially for the overall tone of the series. An honorable mention goes to Mayuri's Tota Pulchra Es, but I feel that almost goes without saying.
For Automne... I'm once again going to set Tota Pulchra Es aside in favor of Memoire. Don't get me wrong, I love the vocal tracks dearly, but I feel like the way some of these background themes resonate with me, with the story, pull everything together so well. That melancholy feeling of days past...
Finally, Hiver, I think I would have to choose Cendrillon. I think it's the perfect lead up to the surprise, especially considering the events it followed. There's a somberness in the strings that I feel sets better to Suoh's mood at this point than to the play itself, and that elevates it for me.
Honestly, I feel like it's a bit difficult to really pick favorites, but I think that despite appreciating the soundtracks as a whole, these themes in particular stand at the front, even if by only a slim margin.