Not actually the worst game ever, but one you were expecting something from and it just fell short, didn't deliver on your expectations.
Years ago I picked up Rune for the PC. I heard it was a Viking game rooted on Viking lore and history. Started out promising enough in a Viking village. Oh boy! I was anticipating some real Viking action, sailing on board a Viking ship, raiding the coast of weakling Europeans, pillaging, looting, storming castles. I got on board my first ship and set sail, all excited about my life of raiding and fighting. Then out of the blue, when we are barely out of the harbor, my ship breaks up and sinks! I float down to the bottom of the ocean and wake up in a cave. The game turns out to be a dungeon crawl in some deep caverns. No sailing, no looting, no fighting, no burning villages, no ravaging women, none of that. I was so bummed I couldn't play the game more than a few hours, long enough to realize it wasn't going to be the game I was expecting it to be.