That might sound too simple, but LCD is bad at black. OLED is great.
Keep in mind many LCD's are backlit with an LED lightsource. That is not the same as OLED.
Others have pointed out that only Sony and LG currently offer OLED, so don't be confused by thinks like "QLED" from Samsung.
There are some really good non-OLED sets, for sure. But if image quality is your concern, OLED wins.