Final answer:
Sir Lancelot is the character known as Arthur's nephew who sat next to Queen Guinevere and is reputed as the greatest knight in the legends surrounding King Arthur and the Knights of the Round Table.
Step-by-step explanation:
The character who sat next to Queen Guinevere, is Arthur's nephew, and is known to be the greatest knight is Sir Lancelot. Sir Lancelot is a central figure in the legends of King Arthur and the Knights of the Round Table, often associated with both great chivalry and a tragic romantic relationship with Queen Guinevere. Within the Arthurian legend, Lancelot is depicted as an exceptional knight renowned for both his bravery and his ill-fated love for the queen.
In Alfred, Lord Tennyson's poetry, such as 'The Lady of Shalott', Sir Lancelot is depicted with vibrant imagery, highlighting his status as a bold and shining example of knighthood, capturing the imaginations of readers with his aura of valor and romance.