We know that human populations have been mixing throughout history, including with species that no longer exist.
We (Homo Sapiens) have genetic adaptations for speech, so it seems likely (but unproven AFAIK), that some Ur proto sound communication immediately fractured on migration, with isolated groups diverging rapidly, such that languages were not mutually comprehensible. Languages are cultural, so mutate much faster than genes. It seems unlikely that genetic adaptations for speech could evolve independently in different populations of our species.
So my expectation is that all languages could be described as creoles, and only the length of isolation since last contact and exchange is in question.
Some might call this purity, but it should not be called that, because it hitches a ride to genetic purity, and we know where that leads. Also pure does not mean best. It just means most unchanged, which often means less successful and more brittle. Creoles are antifragile.
It is often the case that creoles are more useful (successful) than pure languages. Creoles occur at points of contact, trade, exchange, invasion, colonization, enslavement, expansion and diffusion. Development and populations tend to benefit in those situations, so creoles are probably more likely to be useful and survive.
English is not the simplest language. It is certainly a mongrel, if not an academically verified creole.
English does not have a standard. Famously, the first serious and comprehensive modern dictionary (OED) documented actual usage, not imagined correctness.
By contrast, French, and most European languages, have a state-sponsored Academy to rule how the language should be used [1]. Central planning never works. English has evolved through many epochs of tribal conflict, invasion, conquest, colonization, trade, economic success, diasporas, intellectual influence, literature and modern media.
Ironically, it is more of a lingua franca than French. Notice how English beautifully and flexibly adopts a foreign
phrase for a foreign pidgin trade language, which did not include English :)
Many distant historical languages only survived by fervent religious preservation, rather than practical usefulness. Vernaculars diverged, as expected. Latin and Sanskrit come to mind. Priests self-perpetuated their influence, by ensuring they were the only people educated in an obscure dead language, who could read and interpret holy books. A nice rent-seeking grift if you can keep it going for 2,000 years (Catholic church; 500 years earlier for Protestants).
So the more interesting question does not involve the word creole, but instead asks: what are the current languages with the longest period of isolation?
[1] Not coincidentally, the law is the same. English Common Law says you can do anything reasonable, until someone complains, then judges resolve the case, and it becomes a precedent in the common law. But the French have a top-down Napoleonic Code, which tries to describe everything that's forbidden. Needless to say, thanks to the war-mongering megalomaniac Little Corsican, most of Europe has the same legal framework (and also drive on the right, because of Napoleon+Hitler - a remarkable double-punch combination).