Lithuanian is very conservative in some aspects, and very progressive in others. For example, it has lost the neutral gender and it has innovated some cases that didn't exist in PIE. There is no scientific, consistent way to call it as a whole more archaic than any other language.

> Lithuanian is the most archaic language in Europe

By what metric?

> which is what makes it so good

By what metric?

By most archaic language they are probably referring to Lithuanian being considered closest to Proto-Indo-European in terms of language features: https://en.wikipedia.org/wiki/Lithuanian_language

I’ve heard this and also heard it’s mostly complete rubbish. It might technically be the closest but that is still not close at all, simplifying like this doesn’t really help anything.