If it hasn't been proven yet, which source is the truest, then I guess the one which came first should be considered?
Seniority is not without its merits in a world where Christianity has been the state religion for over 200 years. But the answer is also more complex, this is a religion that preserved orally between 797 and the year 1200 and was worshipped all over the place, from Gotland to Greenland so any attempt to create a ubiquitous image of Norse myth is probably going to be a failure. Seniority is however far from the only qualifier when it comes to Snorri who tries to establish a structured afterlife which goes against many other statements, sometimes even his own. One example being his depiction of the goddess Hel, whom Snorri decides as dreary and unpleasant despite the fact that none of her actions indicate that would be true.
As to why the Poetic Edda specificly is so highly regarded is because it's mistakingly thought of as one source, but unlike the Prose Edda it isn't written by one author, or just from one lifetime. The Poetic Edda is a collection of poems that we know are from vastly different time periods and from different geographical locations. We can tell this by the language in use, some words for instance which only appear in texts from the 800's or prior are in there for instance, which shows that the monks who assembled the Poetic Edda didn't actually write it, they just neatly packed togheter a bunch of long seperate sources. This makes it so that every poem in the Poetic Edda is its own source which we know thanks to the authors transparicy of their own sources.
But I'm also opposed to the idea of "the truest (source)", when I try to form an opinion I try to look at a large variety of sources, see where they agree with one another and in the instances that they do you can assume the statements to be true. For instance, lets take my favourite diety in the Norse pantheon, Freya.
"Wrothful was Freya and violently she snorted, the entire Aesir hall beneath her quaked." -
Þrymskviða, the Poetic Edda
"Njords daughter was Freya, she was the first to teach the Aesirs magic." - Ynglinga Saga (Just to give you a quote from Snorri)
"Silent you, Freya! A sorceress you are" - Lokasenna, the Poetic Edda
This gives us a fairly consistant view of Freya, as a goddess of magic, on the other hand her characterization as a goddess of fertility can be traced back only to one sentence.
"She (Freya) greatly enjoys songs of love, she is good to pray to when in romance." - Prose Edda
Based on this I'm drawing the conclussion that Freya is a goddes of (among other things) magic, but her role as a goddess of love and fertility is far more questionable.
After that I think I owe you guys two songs.