Imaginations From the Other Side, then Somewhere Far Beyond. Both are excellent albums. As far as Nightfall in Middle Earth goes, I get tired of hearing people who have no idea what the album is about saying negative things about it or downgrading it in any way. If you have never read the Silmarillion by J.R.R. Tolkien, then you still may be able to enjoy the excellent music on the album, but you can't know the depth of what it means. In my opinion it is one of the best albums I have ever heard, partly because the music on the album is fantastic, but also, because it is written on the skeleton of a history book, and manages to wrap the events and characters in fleshly sinew, so that you can know them. That is no small achievement.
A Night at the Opera so far seems great, I can't say if it is my favorite that they have done or not...I really enjoy it, but it may take another 100 or 2 listenings before I can say which I prefer, bloody that is a musically dense album. I would pretty much, though, recommend grabbing their stuff newest to oldest. They have had the same lineup since they started, and I think that has really helped them to consistently improve and mature in every aspect of their music. I find it almost amusing that they all but disavow their first couple of albums.