Hominster certainly has the edge as far as difficulty goes, but I'd put it in 1 just for the story attached to it. Having to fight the Sin Eater that turned Tesleen as well as Tesleen herself made it feel very tragic. Also the cutscene following its completion is just epic. The only thing Sirensong has that make it memorable to me is the boat. It feels a lot like the obligatory fight during travel as opposed to the introduction to the story.