When I say solid I mean active in exp and pk, and by durable I refer to longevity, server stability and without numerous server wipes. I know there has been some relatively successful PK servers (SexyKO, LifeKO), but I'm inquiring about farm servers (heavy-medium).
Servers I have tried, and would not count among the list are:
Ko4life PS attempt,
All of twostars-related servers (no offence, does great work - simply stating an opinion based on the aforementioned criteria)
I never tried them, but have heard LostSoulzKO and OldSchoolKO had decent runs.
Perhaps there's been some great turkish that never got exposure, who knows? I'm thinking there's no recipe for medium-heavy farm private servers.