Then the safest bet is to choose something that was popular and stable twenty years ago.
https://en.wikipedia.org/wiki/Lindy_effect ("...the longer a period something has survived to exist or be used in the present, the longer its remaining life expectancy")
You *probably* just want Debian. There's many reasonable options but that one's the Schelling point for this question (I think).
edit: Here's the list of Slashdot linux threads from 2003. I think anything flamed in there that's still around is a Lindy candidate.
https://www.google.com/search?q=site:linux.slashdot.org/stor...