What was the first significant DC Universe Animated Original Movie and what made it stand out?

The first significant DC Universe Animated Original Movie (DCAUOM) is widely considered to be "Superman: Doomsday," released in 2007. It stood out as it was the first direct-to-video film in the line and directly adapted the iconic "Death of Superman" storyline from the comics, showcasing a willingness to tackle mature and pivotal comic book arcs in animated form, a departure from some earlier, more standalone DC animated productions.