Everyone who is interested in linguistics knows that Lithuanian is the most archaic modern Indo-European language, that Icelandic is the most archaic modern Germanic language and that Sardinian is the most archaic modern Romance language. These facts are frequently stated anywhere that European historical linguistics is discussed. But what about the Slavic languages?
A few months ago I decided to look for online discussions covering the question of which modern Slavic language is closest to proto-Slavic. This is the most thorough such discussion that I found.
The most knowledgeable poster in that thread seemed to be ahvalj, who hailed from St. Petersburg. He argued for the "it's really, really complicated" position. And he brought up a lot of really complicated examples to support it. Some modern Slavic languages have relatively archaic conjugation systems, others have relatively archaic vowel systems, others have relatively archaic declension systems, etc. There's no non-arbitrary way to say which feature of a language is more important. And proto-Slavic is an imprecise reconstruction anyway.
"As we can see, no modern Slavic language approaches the sounding of the late Common Slavic of the 6th century. Very-very approximately, for purely introductory purposes, I would say it sounded as something between Slovak, Lithuanian and Latvian."
By the way, everyone is very sure that Lithuanian is the most archaic modern Indo-European language even though proto-Indo-European has got to be a much more imprecise reconstruction than proto-Slavic. So one lesson I'm taking away from this is that, with the possible exception of Bulgarian and Macedonian, the various Slavic languages have changed to roughly the same amount since they split.
Then a Bulgarian poster named Christo Tamarin proposed an experiment:
Text. The oldest texts in Slavic are Gospels, presumably of the end of the 9th century. Select an excerpt of a Gospel in Old Slavonic (preserving original vocalization, the older one than Church Slavonic). Avoid commonly known texts (such as Matthew 6:9).To which ahvalj replied:
Speaker. Assign a person who can read the selected text. Should not be native Slav. Make an agreemant about the exact pronunciation keeping it as conservative as possible. Also, make an agreemant about the speed of reading.
Public. Native speakers of all modern Slavic languages which are considered. Exclude those related to the religion (they could know the text by heart). Exclude those related to lingustics. Exclude those fluent in more than one Slavic languages.
Experiment. The speaker reads the text. Each person of the public writes down the translation into his/her native language.
Appraisal. Exact translation will be appreciated and scored.
My expectations. The Russian team wins. Russian is the most conservative. The Macedonian team qualifies last. Macedonian is the most innovative.
Russian is conservative in the sense that it has largely preserved the Church Slavonic vocabulary, so indeed, a Russian speaker will win in your experiment, but other aspects of the Russian language will be averagely advanced, some more, some less, plus the Church Slavonic vocabulary is not Common Slavic, and a great deal of these words never existed outside the Orthodox tradition.
I was surprised that Russian was even in the running. I guess I expected Polish to be the most archaic. It probably had the least amount of non-Slavic influence upon it historically.
I didn't take into account the conservational effects of Old Church Slavonic liturgy (Polish liturgy was in Latin) and of Russia's remoteness. Why are Icelandic, Lithuanian and Sardinian so archaic? Because they are remote. The medieval Novgorodian dialect was archaic for the same reason and I guess that logic also applies to Russian in general.