In all fairness, all of the Nine Cauldrons were very powerful. Although each of them had its own merits, obtaining any one of them could dramatically enhance one's capabilities.
However, if he had to say which one was the most powerful, Jiang Fei would absolutely pick the Cauldron of Luck!
The Cauldron of Luck had three special characteristics. The first one made all attacks critical. Regardless of whether it was basic attacks or skills, all attacks would be critical hits. This would lead to a substantial increase in a player's damage output.
As for the second special effect, the owner of the Cauldron would be able to dodge almost any attack. As long as the opponent's weapon had no special effects or the skill was not a sure-hit type of skill, Jiang Fei would be able to evade it. All attacks would be "luckily" dodged away by Jiang Fei.
