I prefer a mix of both. Enough to self-insert, but also enough for the character to feel fully engaged and moving along the story. If the MCs personality is completely customizable, I find the story falls a little flat, unfortunately :(
I prefer when the MC has personality because it makes the options for how they can behave on a given route feel more real. When they don't have a personality, they feel flat, like a blank slate.
I prefer MCs with set personalities because I don't self-insert, and I like them to feel more real when I read. If the MC is "too bland", it can get pretty frustrating.
