I am a fan of this band. Century Child is my favorite album from this band, followed closely by Oceanborn. Once was a major disappointment for me, as it's too industrial, too much of the crappy male vocals (Marco is not nearly as bad as Tuomas though), and the songs are a little too simplistic for my liking.
I think Century Child was a cool mix of heavy guitars and symphonic atmosphere, while Wishmaster and Oceanborn are incredibly symphonic and have Tarja's best vocal moments.