FF6 - Brave New World

From the comments I've received, the consensus is that the White and Brown dragons are the hardest. The rest tend to vary greatly depending on when in the game you choose to take them down; the Purple one tends to be a royal b*t*h by sheer virtue that it's the first one you fight, while the Gold and Green tend to be easier since they come last.

The only other factor is that the ones representing the "main" elements (Silver, Red, Gold, and Green) have more HP than the others. This makes their fights take longer to make up for the fact that their elements (with the exception of Green, who attacks mostly with status effects) are easier to defend against.