What is considered the most authentic illustration of Yggdrasil? Is there an etching or drawing of Yggdrasil, the Norse Tree of Life, that is the oldest, or considered the most authentic? If so, what information was it based on?

Some Norse myths are depicted on rune stones or other objects, but I am not aware of anything like that for Yggdrasil. The oldest depiction I know of is an illustration in a 17th century manuscript of Snorri's _Edda_ (can be seen here), so it's quite a bit younger than the actual myth. Snorri's _Edda_ is also the source that gives most details of the tree, even if it is also mentioned a few times in the poetry.