Best spec with least downtime so far for me: Demo DA glyph / 20% hp heal on kill. You can't beat that.
1) Go in DA mode
2) GoS your imp (dispel), +30 dps, +20 hp with soul link talent
3) Pull every mob around you with corruption
4) Aoe the crap out of them with your instants and always stay full life with the glyph
5) Demon leap to next pack. Rince and repeat