You really can't go far wrong with FC, maybe because they're comfortable sticking to the sound they do best. I'd could recommend every album but I'd particularly single out Eternity of course, then The Circle of Life and Dimensions is pretty underrated IMO. The latest also worth a spin it's a grower.
I started with Crystal Empire - that's a really good album, I think. I also really liked their new album Legend of the Shadowking but I know that's received mixed reviews from the fans. I recommend the songs Freedom Call, Thunder God, and Out of the Ruins.
I could use some recommendations for other albums too. I'm guessing Eternity is a good one to get since you like it?
Eternity is their best IMHO, but I also like the debut Stairway to Fairyland A LOT. I remember the first time I heard it. Sort of a Magic Never Dies moment.