Descriptive with all the necessary fields to fill in, along with a comprehensive backstory. I know people like their character history to be revealed in game, so I have those players fill out a generic background (still of acceptable standards) as well as PMing the GMs the portion they want to keep concealed until later.

The main reason for this, other than promoting a consistent backstory that can be referenced (raise your hand if you'll remember your character's parents/ mentor's name and where they learned their minor skills without having something yo reference six months down the road), as well as serving as a platform where players lay out the groundwork of how their characters became the people they are and how they learned their skills. 

I am a pretty lenient person for stretching believability as long as it's consistent and there's a logic to it. Tell me why and how your character is a miserable cold blooded murderer who knows advanced space magic and has the ability to entrall people by spitting on them in a way that sounds reasonable and makes sense, and I'll generally let you go with it so long as it's setting appropriate and you seem to have put a lot of thought into it.

The other big old reason I want a detailed character bio is to both provide a writing sample and to ensure you cannot pull a fast one and claim something about your character that was never discussed or approved. For instance, say we're doing a generic fantasy RP and your character suddenly has a pet dragon. I demand to know what the hell you're trying to pull, and then you say when your character was a child and his parents were killed, a dragon took him in and raised him. He also has dragon blood now and can live for ten thousand years.

See where it gets a bit shitty when people are coy about submitting a detailed bio? Look, I get it, it's a pain in the ass and I don't always like doing it either, but it's pretty much the only way you can stop players from pulling dumb shit from under you.

If the GMs don't know about a character's history or hidden skills or whatever, then the player can turn it around and say they were never told that it was unacceptable. You're putting out fires instead of preventing them.