I think there are a number of aspects that are highly interesting about Odysseus and his voyage into the underworld in Book 11. Firstly, it allows Homer to make lots of links between the action of The Odysseus, and his other epic, The Iliad, by focussing on the fate of a number of major characters from that epic, such as Achilles, Ajax and Agamemnon. Likewise we are presented with a number of characters such as Tantalus who present us with a series of punishments that we are shown could await us in the Underworld. Lastly, however, and perhaps most importantly to Odysseus, Odysseus is given information to help him in his future quest.
This information comes from the blind prophet Teiresias, who specifically tells him to stay well away from the cattle of Helius on the isle of Thrinacea. Teiresias makes clear the consequences of disobeying his advice: the companions of Odysseus will be killed and Odysseus will suffer many trials and hardships, returning home only after a long journey and on a different ship that is not his own. Likewise Teiresias also points towards future quests and voyages that Odysseus would undertake once he had finally made it back to Ithaca and restored his order. Teiresias also explains to Odysseus how he can communicate with any shades that he permits to drink the offering of blood that he has with him.
So, considering all of the encounters that Odysseus has, it is clear to me that Teiresias is the most important encounter, because he provides Odysseus with concrete advice and information about the future and how to survive it.