Well, first big needed piece of information is what your spec is, otherwise the rest is extremely variable.
Second, and I can't say this enough (don't want it to get lost), the strongest DK tank will not follow a blind rotation, they'll be continuously selecting moves based on the situation and a set of priorities.
1.) Diseases are up. Blood and Unholy should have both diseases up, at all times, whenever possible. Frost can get by with only Frost Fever depending on what you're doing, but for optimal threat especially building to a long fight, you'll want both diseases as well.
2.) Single target vs Group threat will determine which moves are most effective. For example, Unholy you need to know how many enemies it takes (remembering you need 3 diseases up as well) to make BB better than BS, also considering if you NEED more focused threat or AoE threat.
3.) What is the most effective use of your runes? DnD is good threat, but it is also expensive. Will it add enough damage or can you support the gradual threat to make it worth the cost, can you place it ahead of the dps start enough to get the runes back more quickly when you're really working the threat?
So, if you share your spec I'm sure there are folks who will offer some rotations that work for them, and I'd be happy to sketch out some priorities.