Completionist Cape is the hardest cape to obtain. In order to get this Cape, you need 99 in all stats (except construction) as well as level 120 Dungeoneering. You need to have finished all The available quests. On top of those skills, you need 500 player Kills!
How to obtain the Completionist CapeEdit
After you have the requirements to purchase a Completionist Cape; head to lumbridge. Just north of lumbridge castle, next to the general store Prince Brand will be wandering around. You have to have your stats completely restored in order to purchase a cape!
If anyone has the stats for the comp cape - please send me them! As well as a picture of wearing just the cape, thanks!