Assassins Creed Unity
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To release this as the sequel to Assassin's Creed IV: Black Flag was the most disappointing experience I've had with gaming. From clunky mechanics to a story that doesn't bear remembering, it was the first missed step on the flight of stairs Ubisoft pushed the Assassin's Creed franchise down.
Honorable mention goes to Cyberpunk 2077 on release. Paid full price and was hyped only to be horribly disappointed by the performance. It was completely unplayable at that time. Went on to become one of my favorite games of all time though.