My favorite album is Origin because I just love the dark atmosphere of it. The songs were catchy yet mournful and creepy (Where Will You Go, Lies, Even in Death), and there was an almost "romanticized" feeling in some of the tracks which I found hauntingly beautiful. And Eternal just happens to be my favorite instrumental - I love the piano and the thunderstorm in the background was soothing.
It's really hard to pick. Every album is special in its own way to me. It's just like picking a favorite among a collection of amazing tracks. So, every album is my favorite. :Q