I love any metal that incorporates a symphonic sound. That's why I like a lot of black metal, bands like Dark Throne.
Also, speed/power metal like Blind Guardian is awesome, for the same reason.
And I love straight power metal, like Hammerfall. A lot of which is quite mellow.
I love Type O Negative too, the song Everything Dies was brilliantly done, with the piano.

Another really mellow band is Agalloch. I've never met anyone else who's actually heard of them, but they're like an acoustic version of Opeth; acoustic doom metal. It's great.