Who acted Tom Riddle in HP 2?

The actor that plays Tom Marvolo Riddle is Christian Coulson in Harry Potter and the Chamber of Secrets.

Unless you are refering to the actor who plays the adult Riddle (Lord Voldemort) who is Ralph Fiennes.

The Voldemort in the first movie was the face of Richard Bremmer and the voice of Ian Hart.

Frank Dillane played him as 16 in the Half-Blood Prince and Hero Fiennes (Ralph's nephew) plays him as a young boy at the orphanage.