fwiw I didn’t want to suggest Dele Alli as a flop, he was a genuinely brilliant player for a few years. I only mentioned him as another example of guys undone by something mental, though for Balotelli that appears to have been simple brainlessness, hence him seeming to find a comfortable level only a couple of steps down.
For flops I’m looking for players who never reached their supposed heights at all. Another kind might be players who made bad moves so we’ll never know how good they were. Axel Witsel, he looked amazing but he chased the money and then never played for an elite team afterwards. Probably plenty of guys who moved to City or Chelsea and got buried.