Not usually a fan of unplugged, but there is one that I like quite a bit.
That is Pain of Salvation - 12:5.
These guys are one of the best progressive metal bands form Sweden. Their 'plugged' releases are full of emotion, complexity, world class musicianship.
They did this one unplugged release back in 2004. Great acoustic guitar playing, harpsichord, brushwork on the drums, and Daniel Gildenlow's emotion filled vocals. The harpsichord gives it a certain 'chamber music' feel without losing much of the metal power.
The cool thing about this, is that they just didn't play the original songs 'unplugged', but they rearranged them and added new parts. For example, the song 'Ashes', is originally a very dark song played in a minor chord. But in this reworking, the change it to a major key, giving it an almost gospel feel.