I wouldn't recommend starting with Nightfall or A Night At The Opera, because they might be a lot to digest at once. Especially Opera, which is just crazy over the top with the overdubbing and layers to the songs. I'd recommend Imaginations From The Other Side myself-it's a good midway point between their initial, straight up power metal and the more orchestrated music they did later. If you like that, you'll like both their earlier albums and will be ready for the more orchestrated ones.