There are five royal palaces in Seoul and, although Gyeongbokgung Palace is the largest and most visited, it's Changdeokgung Palace that has been listed as the World Heritage Site. That's primarily because of its authenticity, as the one that is in the most original form and the first to be restored (by 1609) after being razed in the Japanese invasion. It was the seat of government for 270 years until the very end of the Joseon Dynasty.

My sense is that it's actually hard to tell the difference between the centuries-old restoration here at Changdeokgung and the more modern work at the other palaces. In a way that's good - they all 'look' authentic. But I find it a bit disconcerting because the Korean World Heritage Sites have a tendency to all look a bit the same and seem a bit too clean (to the point of almost being sterile). That's my main criticism here - that the buildings are all well maintained but it's hard to get a sense of how they might've been used or what life in the palace might've felt like.

The highlight at Changdeokgung Palace is probably the Secret Garden, with beautiful landscaping that follows the natural contours of the land and is interspersed with various pavilions and other buildings. You can only access it with a guided tour and I definitely recommend doing that. In busy months, it's worth booking in advance, although there are places that only become available on the day (a good reason to come early and secure a spot). 

I would recommend buying the Royal Palace Pass at the ticket counter, which costs just 10,000 won (approx US$7.60) for adults and includes Changdeokgung Palace and the Secret Garden, along with Changgyeonggung Palace, Jongmyo Shrine, Gyeongbokgung Palace, and Deoksugung Palace.