How fierce was T.rex compared to Velociraptor and who would have won a fight between the two?

In terms of pound-for-pound fierceness Velociraptor may have been marginally more fierce. In the unlikely event of a fight, a single Velociraptor would have made no impression upon a T. rex simply because adult Velociraptor grew to a length of about 1.8 metres and stood no more that 1 metre tall, while T. rex reached lengths of 13 metres and stood 4 metres tall. It is just conceivable that a large pack of Velociraptor might have harried a very sick tyrannosaur, but a straight fight between the two would just have been no contest.