Which giant's eyes did Odin place in the sky as stars?

The giant whose eyes Odin placed in the sky as stars is Thjazi.

In Norse mythology, Þjazi, commonly anglicized as Thjazi, was a powerful jötunn associated with the mountains and winter. After Þjazi was killed by the gods, Odin took his eyes and cast them into the heavens, where they became stars. The episode appears in the mythological material summarized by Snorri Sturluson, though surviving Norse sources preserve many stories in varying forms.

Thjazi is especially famous for abducting the goddess Iðunn and her apples of youth. Without Iðunn and the apples, the gods began to age. Loki was forced to help recover her, eventually transforming himself and Iðunn so she could be carried back to Asgard. The gods then burned Thjazi when he pursued them in eagle form.

This story is easy to mix up with other Norse explanations for stars and constellations. Ymir, for example, is the primordial being whose body was used to form the world, while Hræsvelgr is a giant associated with an eagle at the edge of the world. Neither is the giant linked to Odin’s star-making act.