I'd have to say Vault. The mystery and teamwork and overall dynamic of the gameplay was stellar. Not only did you have to kill and survive, but you had to coordinate and solve puzzles. Nostalgically the rewards were some of the best at the time and even now, VoC and Fatebringer, among others, are still relevant and valuable rewards. That chest though ( ;) ) ! And Beating Atheon you felt like a Hero! Crota's end had too much cheese, despite the OP rewards, but the material system really sucked having to farm those ( some weeks you complete hard mode and barely get any materials wtf). That raid required little team work in my opinion until the end (especially because of skipping the bridge). At least hard mode was challenging... At first. PoE feels dull in comparison to the complexity and fluidity of the raids, despite the etheric light. The HoW stuff is just terrible in my opinion. And nightfall just didn't get me excited, even though I play them more than all of the other activities. Perhaps nightfall is on par with the time-to-reward ratio, in addition to doubling up experience and bounties with it. But nowhere near the excitement of Atheon.