for me, it's:
Although the top 4 are very very close, I actually wouldn't mind any of them being my GF
Kohaku, I like her as well, but it's more out of sympathy than love.
Anyway, I find all of the Tsukihime girls very very charming in their own ways. At the end of every route, I always wanted them to have the greatest happiness.
Arcueid - Cute, naive, moe and loving. If it was her, I wouldn't mind her sucking my blood. Reminds me of Saber in Fate.
Akiha - Ultimate little sister you can have. Cute. Tsundere. Loyal. Always puts her "brother" before herself in any situation
Ciel- Very loving girl. Always gives assistance to Shiki whenever he needs it most (in most of the routes without her, there wouldn't be a good ending). Also, her
sex with Shiki was just so hot.
Hisui- One word, tsundere.......... She is the type of girl whom you just want to look after and hug
Kohaku- I actually like her, despite what she did in the plot. She thinks herself as being emotionless, but once you can go beyond her barrier, it is totally different. Of course, her sympathy vote just add to her points