My heart will always belong to Wishmaster, there is just something about it that no matter what mood I am in it lifts my sprits. Oceanborn comes in a close second for me because I connect personally to so many of the songs and some of my favorite songs come from Century Child, but I have to agree with Zelda, it is one of the few that I actually skip songs on. Once is pretty special to me as well since I was in Europe right after it was released and it was the first NW album I ever purchased I remember seeing the rack full of them and thinking wow you are lucky if there is 2 or 3 at a store in the states. Every time I listen to Once it takes me right back to London and Paris.
Pretty much each album has a meaning to me and I could say something about every single one of them.