Shikieiki, because being able to hold my own with her in a conversation on morality is really appealing.
Sanae, because she reminds me of everything I loved in my best friend/girlfriend in high school, and she just seems like she could use a best friend from the outside world to be a fangirl with.
Koishi, because she's awesome. Though, I'm biased, since she's White Rose's main character. :V
Satori, because she seems quite friendly. In fact, she's starved for company. I would love to hang out with her.
Keine and Akyu for the same reason: I love history, and I love to hear stories of times past. They would love an audience.
Ibara Kasen, because I'd like to know what it takes to be a hermit myself, and because I'm just plain curious about her.
Definitely Aya, because... well, she's Aya. I would love to talk with her about all the interesting things she sees in Gensokyo. Hatate too, since she's even funnier than Aya. An argument between them would be incredibly funny to sit in on.
And out of left field: Tenshi, because befriending her would be a whole lot of fun; there'd never be a dull moment with her.
As for who I wouldn't like to meet? The ones that stand out are Yukari, Remilia, and Nue, since they are (in varying degrees) dangerously whimsical and downright annoying, not to mention insanely dangerous in their different ways. Granted, hanging out with Tenshi is still very dangerous, but funnier too.