Which disciple doubted Jesus' resurrection?

Answer: Thomas

Thomas said he wouldn't believe unless he saw.

“Now Thomas called Didymus, one of the Twelve, was not with the disciples when Jesus came. So the other disciples told him, “We have seen the Lord!” But he replied, “Unless I see the nail marks in His hands, and put my finger where the nails have been, and put my hand into His side, I will never believe.””— John 20:24-25 (BSB)