I see everyone is going to jump on the Rated R bandwagon now.
Wasn't the first Spawn R Rated? As was Blade, and Sin City
First Spawn was PG-13, but it was supposed to be R and was toned down. Blade and Sin City were R. The Watchmen was R as was Kick-Ass. Deadpool was just the most successful R movie.
I believe Joker has taken that crown from Deadpool.