I'm still sitting on my second best friend unsure of what to do with it.
On one hand I'm inclined to use it on someone I know personally (Matsuri and I discussed this some, and I don't necessarily have a problem with that) because I know they're unlikely to just vanish.
There's some merit to picking a seemingly-reliable rando who mains something that I use a lot. I have one hypermax Neptune friend and two Blodins, and having access to any of them all the time would be nice. But since I don't know them it's kind of risky. If I could find a hypermax U/W Sun Quan I would BF them in a second, but still seemingly none exist.
I'm also kind of considering waiting and hoping to find someone who can help me with my "clear as much of the game with a terrible Evangelion theme team as possible" project. If I ever find a hypermax Misato or ultimate combo golem by some miracle it would help me tremendously to best friend them. But again, it's risky and unlikely to ever happen in the first place.