What significant ghost does Odysseus speak with in the land of the dead?

The most significant figure Odysseus encounters in the underworld (Book 11 of the Odyssey) is Achilles, the central character from Homer's Iliad. Achilles, a demigod, was the greatest Greek warrior. He killed Hector, Troy's most prominent warrior, as revenge for the death of Achilles' friend Patroclus. As was prophesied, Achilles later dies and becomes prince of Hades. Unfortunately, Achilles--cavalier toward death while alive--now wishes he could return to the land of the living. As he tells Odysseus:

“I’d rather be another’s hired hand,……than to rule over all whom death has crushed.”

Another significant ghost Odysseus encounters is that of Agamemnon, the former king of Argos. After winning the Trojan War, Agamemnon had returned home, where his wife--Clytaemnestra--and her lover--Aegisthus--murdered him (The Greek playwright Aeschylus narrates this event in Agamemnon, the first play in the Oresteia trilogy). Due to his fate, Agamemnon cautions Odysseus against placing too much trust in his wife, Penelope, when he finally returns home to Ithaca.