I'd say all albums... except Once, because that's a masterpiece album.
I always found Oceanborn to be the best. Every track on it (with the exception of #3) is brilliant. Century Child is my 2nd favorite Tarja album. Once would probably be next, although I see why many rank it as high as they do.
Yeah, it's pretty good, but I've been hitting recent Epica albums, and absolutely in love, especially the unplugged + retooled versions of their songs on their latest album.
I don't have any of the new ones. I should change that.