If there was actually a devil that came and asked me such a question and proved who he was without a shadow of a doubt, I'd probably refuse his offer.
The notion of a Devil being real probably means a place like Hell exist and while I'm by no means a good Christian, I'd rather not be stuck in pain and Hellfire for all eternity.
If I did have to make some deal with him though, it'd be for life everlasting with the ability to manipulate my body and age at will.
Prevent myself from aging into an old man and the ability to change my age and youth.
That way even if I existed for all eternity and everything went into nothing, there'd all be some way for me to adapt to survive.
My goodness, that got a little too deep into thought, lol.