Lights that use watch batteries (e.g. CR2032) are not for heavy use. They don't cast much light, and while the battery may last a while, they cost a lot per milliamp-hour, so the fuel cost is pretty bad unless the light is just for occasional use.

"Just buy this" doesn't seem to reflect a lot of research. There is no one-size-fits-all bike light, and we don't even know the original poster's particular needs.