In what general situations or flight scenarios is turbulence most dangerous to aircraft? When could it be bad enough to bring down an aircraft (even up to something as big as an A380)?
Wake turbulence can affect aircraft in the most critical phases of flight such as landing approach.
For example, consider this AOPA report
On June 12, 2006, while on visual approach at Kansas City International Airport in Kansas City, Mo., the pilot of a Piper Saratoga crossed below the flight path of a Boeing 737 that was landing ahead on a parallel runway. The Saratoga encountered wake turbulence so violent that it tore apart the aircraft in flight. The pilot and his passenger were killed.
The effects are most severe for small aircraft entering the wake of a large aircraft but large aircraft can be affected too.
Skybrary lists several examples of incidents involving wake turbulance.
Turbulence is most dangerous when landing, or when it is extreme.
A sudden downdraft during landing can cause a plane to crash, notably the Dallas crash of Delta 191, in which most of the people aboard died.
Inside of storms turbulence can become so violent that it is threatening to the structural integrity of the aircraft. Although planes can and do fly inside of hurricanes and other violent storms, if the pilot makes a mistake and gives the wrong input, control surfaces can be damaged or destroyed.
Turbulence can often be quite violent near mountains, downdrafts produced by rotors and lee waves being particularly dangerous. Expert pilot Steve Fossett was killed in that way. Even large planes have been destroyed by mountain winds such as BOAC Flight 911.
Turbulence in combination with a low-visibility condition, such as a storm, can also cause a pilot to become disoriented.