Unholy Deathknight.
It has decent self healing and defensive cool downs.
It has a gap closer, snares, an interrupt, and a silence.
Gargoyle is a great damage CD.
Its spell composition rewards a highly aggressive and near tunnel vision play-style.
Relatively little macro usage compared to something like a warrior, paladin, or hunter to start being fully effective.
Great for bgs, and has access to great comps ex: for 2s dk/healer & 3s TSG.