If you like a thrashier type sound, you can go for The Dungeons Are Calling, Sirens or Power Of The Night.
Their one misstep was the commercially aimed Fight For the Rock but it is actually not bad overall for what it is.
Their progression towards symphonic rock mastery began with Hall of the Mountain King so you are on the right track there.
Then came Gutter Ballet and the first of four concept albums Streets: A Rock Opera.
Handful of Rain is a good album that was written in the aftermath of the death of guitarist Criss Oliva.
The 2nd concept album is Dead Winter Dead and the third is The Wake of Magellan.
The fourth concept CD was the weakest in story but a solid album musically. It is called Poets and Madmen.
You can't go wrong with any of them.