Possible Oldest Dialect
Simon Roper, a linguistics enthusiast and knowledgeable gentleman on Youtube , posted a video not too long ago regarding whether or not there such a thing as an “oldest English dialect”, that is, one that retains the most archaic phonetic, syntactical, grammatical and/or vocabulary-wise features in contrast to other varieties of English.
How even does one gauge this?
Comparing it to Standard English, whose basis is that of RP, or Estuary English, which has been the prestige dialect ever since Chaucerian times. This, however, is a misnomer, since one would have to compare to a manner of ancient dialects within the scope of Old and Middle English epochs, thus then comparing which vestigial features have survived to our days and which have not.
Even then, as Roper mentions the Cumbrian dialects has the unique plural of house as housen, which however does not follow Old English grammatical patterns since the plural back then was uncountable, i.e. hus, where its modern Cumbrian -n plural is a product of later patterns, which in itself is of old stock, albeit not 8thcentury ones.
Thou
Perhaps the single most iconic trait of popularly perceived archaic language is that of the second person singular pronoun, thou, whose status as an arcane pronoun is very much steeped in religious and Shakespearean language, thus some have confused it for a reverential pronoun, where it is in fact the informal one, corresponding to German du and French tu.
In the northern dialects around the Cheshire area and thereabouts the pronoun still has the meaning of an informal pronoun versus that of ye/you which since Elizabethan times had been used as a marked of formal or respectful speech, where we nowadays in most varieties of World English use honorifics such as “sir” or “madam.”
The pronoun survived, or rather, continued to live on despite its disappearance from mainstream English literature in the subsequent Georgian times, when Elizabethan times had seen the informal pronoun wane in usage due to egalitarian society preferring the formal “ye” and later its object form “you” as the urbane choice, where “thou” had acquired connotations of insult or extreme intimacy.
Northern English phonological developments saw the pronoun morph in pronunciation to tha, /ðei/ - essentially homophone with "they", though the object form, thee, would see predominant usage in more recent times and even be used over the possessive cases thy and thine, since technically this was a merger in pronunciation of both the object and possessive cases.
It has been waning in modern times, i.e. ever since the 1950s, and it now lingers on with the older generations, though it has seen occasional usage by younger ones such as seen in Leeds-based indie rock band, the Kaiser Chiefs, whose lyrics frequently use thee as a direct pronoun of sorts.
Can it be termed archaic or "oldest" by this variables? Relative to Northern English dialect speakers it is old, certainly, but by no means a part of some far ancient past as much as what their grandparents or senior relatives use or used. It is a trait that is slowly dying out, but nevertheless one that is part of modern times.
Looking yonder
In other countries we see traits that would or could potentially be termed "archaic" when contrasted with the standard written mode of speech, literary language: Danish dialects such as the Jutish ones feature the first person pronouns A and Æ, both of which are derived from ak the Old Norse progenitor to the Standard Danish jeg (in its medieval guise as jak), nevertheless dialect speakers will not see it as archaic, but merely part of their linguistic present.
My own grandmother would be asked on occasion, "do you say jeg or a?", to which she would adamantly reply "A sæyr jeg" (A say jeg), which furthermore underlines how natural it is as a part of her own language, as much as other ordinary pronouns such as jeg would be in Standard Danish. Again, it is wholly a relativistic experience.
The term dialect is even a oscillating one and part of a political rather than linguistic considerations.
The Scandinavian languages of Norwegian, Danish and Swedish taking it even further exhibit words that in one language is considered archaic and in the other merely its regular word for a concept, i.e. evening - "kväll/kvæld" versus "afton/aften" and autumn - "höst/høst" verus "efterår". Distinctions that are the product of several hundreds of years of separate and independent linguistic developments.
European Spanish and Latin Spanish has seen the Latin second person plural and later second person singular formal pronoun vos develop in vastly different directions, one reversing its usage as a formal pronoun and the other rendering it to serve as an even more reverential case.
And the list could go on, this is of course only touching upon the topic of vocabulary, then there is the question possible archaic syntactical and phonological traits.
It is an interesting question that would be worth looking deeper into, in terms of ancient languages and their relationship to the varieties back then.