What element has the most similar properties to sodium?
Chemical "properties" are based on their electron structure, and it is how the Periodic Table of the Elements is constructed. The columns (Groups) are elements with the same valence (outer) shell electron structure, increasing in mass as you go down a group.
So, the elements with similar properties to sodium (Na) are all of those elements in the same Group. The ones "most" similar would be the ones closest in mass as well. Those would be Lithium (Li) and Potassium (K). Of those, I would consider Lithium to be "closest" as it shares the higher reactivity with sodium compared to the heavier elements in the Group.