Merlin
The Hat shivered as it realised just who was sitting beneath it. Even without access to the man's memories, the raw magical energy coming from him was a dead giveaway.
'Um, sorry to interrupt, but could you please make a decision already?' the hat hesitated; it would be rude to deny the request of such a man but …
'I can't,' it felt the disbelief and continued, 'the one defining characteristic you have is magic, you are born and bred of it, and there is no house that caters to that.'
'So once again my "destiny" keeps me from living a normal life? I cannot even attend a school properly I'm so strange?' the voice was polite on the surface, but underneath it sounded irritated and tired.
'I'm afraid so. Your magic is just too strong for any of the normal houses characteristics to have much sway on the decision.'
'So what do I do?'
'I don't know,' the hat said softly.
The school was very surprised with the length of time the hat took to sort the boy, Emrys, into a house; there was a rumour that he had broken the record for the longest sorting. Eventually the hat seemed to make a decision.
'GRYFFINDOR!' he shouted.
No one would ever know that Merlin had chosen that house purely because he liked the colours, they reminded him of home.