Well, for me the band of choice is undoubtedly Therion. It has been almost 15 years since I listened my first song of them, and it has been an amazing experience and, of course, they are my favorite band.
Therion is a swedish band formed on the lates 80's as Blitzkreig, and later changed their name to Therion. At first (the first three albums - the third more or less) they played Death Metal (and some regard them as a pioneers in such genre), and then changed into a more operatic/symphonic metal and in some way the hekped to popularize the Symphonic Metal genre, paving the road to other bands (like Nightwish) and they have merged successfully both Classical music and Metal music. Currently, they are perhaps a little less operistic/symphonic as in previous years, and a little more on the Heavy side, but that doesn't had undermined their creative force.
Something curious about them is that they always are experimenting and trying new musical paths as only few albums in their history sound similar (you can assert this, for example, if you listen at least one song of the following albums "Of Darkness", "Theli", "Deggial", "Sirius B" and the infamous (for mixed reasons) "Le Fleurs du Mal").
And as a sample, the follwing song is one of the best songs (IMHO) and belongs the an album which is located almost in the middle of their current career:
Ginnungagap, from the "Secret of the Runes" album (2001)