I wish Issei from SN was half as good of a character as Arihiko. A minor character in the overall narrative of the VN, but his story/perspective resonates with the main themes of Tsukihime and Shiki as a character.
Every time I research more about Zhuangzi's Butterfly Dream, the Eight Consciousnesses of Buddism, and Dao/Taoism I find myself referring back to Kara no Kyoukai and end up falling more in love with the series and it's themes.
I think the Fate route is pretty good if you haven't seen Zero beforehand or if you're aware of Saber's identity. Not that Zero is bad, but it's a pretty overhated route when you have all the mystery and lore known already considering it's needed to setup UBW and HF.