most dreadful

• terribleExtremely poor or distressing
• horrificExtremely shocking and dreadful
• awfulExtremely bad quality
• ghastlyTerrifyingly horrible or pale
• frightfulInspiring terror or shock

appalling · horrendous · nightmarish · gruesome · terrifying · harrowing · grim · dire · fearsome · monstrous · abominable · atrocious · hideous · unspeakable · chilling · blood-curdling · macabre · sinister