As stated the best medium is in fact the Witcher Griffin set obtainable at level 8 roughly. This is because it has a set amount of Sign Intensity bonuses that other medium armor you find may not. And once you hit level 14 and so on it upgrades further.
Your choice would be to find a guide online. Or try to find it naturally by buying the appropriate map from a vendor. If your lucky you may also randomly stumble upon it which also starts the quest.