Like Lord9511 siad it's really all based on the situation. Say you have a Gengar and a Cursed Powder attached and your opponent has Salamence ex and a Bagon. You could knock out Salamence, but Spread damage to the Bagon . Then, Salamence ex kills you. Then knock out Salamence ex with Cursed Powder and then kill Bagon for game.
Really though Knock Out's are best. Thats really the whole point of the game. Though it's so original, that everyone is prepared for a KO. Although some KO's are a really big suprize.