Screenplays are written in 12-point Courier, or a slight variation (Courier Prime, Courier New, Courier Final Draft, etc).
Courier is a fixed-pitch font, meaning each character and space is exactly the same width. Since standard screenplay format is designed so one page equals approximately one minute of screen time, consistent spacing is important.
Most scriptwriting software will default to Courier 12.
If you are an avid script reader, you may come across scripts written in other fonts. Most sold-in-bookstores versions of scripts are this way, and some top-tier screenwriters write in other fonts. For example, many Coen Brothers drafts commonly found online are not in Courier, but as A-list auteurs who write, direct, produce, and edit their own work, they are beholden to fewer people and have more leeway. In this instance, do not follow their example.