Hey all.
I have all the opera metal discs that Therion's done and I'd have to say my favorites are Deggial and Secret of the Runes. I have the Korean version which has 3 extra tracks (though not all of them are operatic - including a cover of Summernight City).
Vovin comes in a close second for me, with Theli probably bringing up the rear.
If you just like the operametal side of Therion though, I would NOT recommend buying the 2 CD live set. It just doesn't do the orchestrations justice and there is a bunch of the older dark/death metal type stuff on there.
Jeff.