The ESRB uses "M" (Mature 17+), not "R." There is no static master list — ratings can change with DLC, and regional differences exist. Here’s how to find M-rated Xbox 360 games reliably.
Use Xbox 360 Family Settings (Settings → Family → Content Controls) to block M-rated games by rating. A passcode is required. Check ESRB content descriptors (Blood, Intense Violence, Strong Language, Sexual Content, Drug Use) for specifics on each title.