Like so many things, these are issues that some of us gladly leave to those with math degrees and generally too much time on their hands, to calculate proc rates, rotations, and overall output. The only good news is that both dps and tanks generally use the same sigils. This is due to the fact that death knights generate aggro through dps output. So basically what you have is a bunch of threat amplifiers, like Death and Decay and Frost Presence, and then the rest is up to your dps.
Frost has it the easiest, alas the best sigil for them is still the Sigil of Awareness. This can be bought cheap with emblems of conquest from your local Dalaran vendor. Since the patch 3.2.2, both blood and unholy now use the Sigil of Virulence, which basically acts like a miniature version of the Darkmoon Card: Greatness, procing your strength by 200 with a very high up time (estimated 80%).
The downside is that the Sigil of Virulence costs 25 emblems of triumph, so it can take many daily heroic quests for you to get it. However, Sigil of Vengeful Heart is still a very good spaceholder until you get virulence. It is sold by the conquest vendor as well, for 19 emblems, and of course it also has a chance to drop from 25-man XT-002 Deconstructor.