Prime Glyphs

Glyphs are an easy way to increase our abilities and utility, sometimes on the fly with some Dust of Disappearance. Often you can come by most popular Glyphs for around 20-50 gold on the Auction House, so it is definitely within everyone's reach to get their Glyph slots populated with something that helps us do what we do best: Getting attention while taking a beating for the group.

• Glyph of Death Coil: If you are specced into Lichborne, this Glyph will increase your healing by 15%, which is not a bad deal. If you are not specced into Lichborne, the value of this Glyph is very limited. Our Runic Power would be better put to use in Rune Strikes.
• Glyph of Death Strike: This Glyph has some situational use. When your Runes are on cooldown, you should be using Rune Strike on every GCD for the additional threat and damage it offers. However, that being said, this is basically free damage to an ability we use a lot, so definitely a solid choice.
• Glyph of Death and Decay: This turns our staple AoE threat from 10 seconds into 15 seconds. With a 30 second cooldown, that means it could be up 50% of the time. While its damage isn't great, the threat from DnD cannot be beat, and is often enough to stay ahead of DPS on AOE fights.
• Glyph of Frost Strike: This has absolutely no use for us as only Frost DKs have this ability.
• Glyph of Heart Strike: Heart Strike is a staple of our rotation, especially on trash with multiple mobs. This is basically a free 30% damage 
• Glyph of Howling Blast: Again useless to us as only Frost DKs can spec into this.
• Glyph of Icy Touch: This Glyph increases the damage of Frost Fever (even applied through Outbreak) by 20%. If you tank with diseases up on trash and use Pestilence to spread them, you may find some minor use from this. But it pales in comparison to the value of Glyph of Death and Decay for threat.
• Glyph of Obliterate: This Glyph is great for DPS if you're Frost, however as Blood we should use our Frost and Unholy Runes on Death Strike, not Obliterate.
• Glyph of Raise Dead: This Glyph is great for Unholy DKs who have a permanent pet. But for the limited up-time of our pets (often to be sacrificed for Death Pact), this is not worth it.
• Glyph of Rune Strike: Great Glyph for Bloody Tanks. Rune Strike is a large part of our rotation, and what we spend most of our Runic Power on. More crits equal more threat.
• Glyph of Scourge Strike: To follow the Glyphs for Frost and Heart Strike, this is the Unholy's signature Strike. Has no value for us at all.

To sum this up, there are five Prime Glyphs that provide value for us, and none of them are really the wrong choice. For running Heroics, I can highly advice the Glyph of Death and Decay and Glyph of Heart Strike for the additional multi-target threat. DPS isn't always considerate on what target to attack, nor do they ever all focus on the same mob.

That leaves Glyph of Death Strike, Glyph of Rune Strike and Glyph of Death Coil, all of which are good options. Though you may get more overall use out of the first two than the latter, which is very situational.