Technically there's no 'best skill' since every skill has their own uses in different situations.
Poison is good for taking out monsters in quests, doing damage over time~ Medium so use in events like arena and battle arena.
Slow is good for pretty much all sorts of events since it slows monsters down and slows all enemy units in terms of PvP.
Knockback is medium because it only knocks back one unit in PvP cases compared to slow.
Area Attack is nice for dealing massive damage, especially good against mobs of monsters.
So slow and knockback are the most widely used.