18 - Treachery & Deceit
Treachery & Deceit (May 2007) - This set was rushed out before the upcoming license expiration, suffered from lack of playtesting, and contained the largest number of overpowered or "broken" cards, for which official errata had to be released almost immediately.
Source: https://de.wikibrief.org/wiki/The_Lord_of_the_Rings_Trading_Card_Game
This set also received the smallest edition of all and is incredibly difficult to find.