All albums with Roy Khan is good, especially Karma, Epica and The Black Halo and the two first Siege Perilous and The fourth legacy is my least favorites but what do I know. The drummer Casey Grillo is a fantastic drummer as well.
You should also check out Khan's old band Conception. There are some really good albums there as well. Parallell minds, In Your Multitude and Flow are great albums. They have Tor Østbyon on guitar from ARK and DC Cooper's solo album.