ARES has won every inter-server war, and does contain the most skilled players overall. this is because ares is the oldest server so many of the experienced players are playing there. though, since it is the oldest server, it is the hardest server to start on since the gear/lvl advantage is so large.
cypher as the guy above me pointed out is, in fact, THE most duped out server on knight online. cypher's market has been fucked since day 1 of its launch as numerous large turkish cheating clans have went there (i.e. HeaveN) and duped unimaginable gear. also, it was home to the most koxpers. Cypher isn't even mentioned amongst most of the ko community anymore and is considered the "forgotten" server as k2 abandoned it.
logos is an underpopulated server along with edana manes and pathos.
i'd the say beramus is also up there in terms of skill as they have produced some notable players such as hitmachine, though each of these players are usually people who have migrated from ares (the original server).
diez is notoriously bad.