Both. Depends on what they are for. Though technically the front crease is a newer style (as in, 1920s-30s), the uncreased front looks more casual by far to me. So, if they are part of a suit or just for dressier occasions, creased; if they are odd and/or more casual or fashion-ish, they could be uncreased as well.
It depends on the material. Cotton chinos without, "proper" wool trousers with crease. The crease in flat-fronted trousers does not go all the way up to the waistband, but stops about 4" below. Not many dry cleaners know that.