Level 60 (Blood Tap, Runic Empowerment, Runic Corruption)
I'm going to be honest, if you can handle using it, Blood Tap is far and away the best choice. For why you should use Blood Tap I HIGHLY recommend reading my article about it:
http://sentrytotem.com/news/classes/...-use-blood-tap
That article is for MoP; in WoD Rune Tap being an important AM tool is yet another reason why Blood Tap is superior since it can more reliably provide an on-demand Rune Tap. Defile also favors Blood Tap if you choose to cast it outside of Crimson Scourge procs; if you are using Runic Corruption, using an Unholy Rune on Defile will 'desync' your runes, which will be a huge problem for you. So if you use Runic Corruption with Defile, I recommend only casting Defile with Crimson Scourge procs.
Necrotic Plague also favors Blood Tap if you choose to not Glyph Outbreak; since it only takes a single rune to refresh, you will have the same orphan rune issue as with Defile using it with RC. For Breath of Sindragosa there is no special preference either way, although BT can be more reliable for maintaining the breath for longer.
The only reason I don't tell absolutely everyone to use Blood Tap is that a lot of people screw up managing the resources of Death Knights properly without capping; managing Blood Tap in addition to that can be way too much to handle for some people. So if you honestly struggle with using Blood Tap, you have two choices; keep practicing until it is second nature, or you can use Runic Corruption instead as the easier choice. Ultimately using Blood Tap badly is worse then using Runic Corruption properly, and it is much easier to play Runic Corruption properly.
For reference on what I mean by 'playing them correctly', here is my mini-guide to using the rune regeneration talents:
http://sentrytotem.com/news/classes/...vel-75-talents
While it was written for MoP, pretty much everything should still apply to WoD.