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.
I prefer MC's with "no personality" if that means I can dictate what they are like in game.
I don't really know if this would count as self inserty but I prefer to create a set character in my mind, with a set personality and stuff then play as that character in games.
I don't mind MC's with set personalities but I typically like playing the most wholesome version of a character possible and not all games have MC's with that set persona. I guess the problem is that it would take a lot of writing/coding to accomodate various personality types for an MC tho.
I hate protagonists without personality; it's so irritating to see it! It's good when there's a combination of choices that determine the protagonist's personality and how she'll act, something that guides the story towards a happy, neutral, or bad ending depending on the choices made along the way.
I prefer when they have a personality, puts a little more of character in the game interactions. Besides is fun when I, sometimes, do not agree with the MC actions and can just call they name