If you're referring to the weapon runes, then Dillbrt is right. If you're talking about glyphs, as in the product of the Inscription profession...
Death Strike's excellent. I personally like the Glyph of Death Grip for leveling (resets the cooldown whenever you make a kill that gives XP/honor), but it's certainly not the most powerful glyph around. Glyph of Disease is always good, as is Dancing Rune Weapon and Dark Death. For minor glyphs, Horn of Winter's almost a must, as is Pestilence.
Here's a list on WoWWiki of death knight glyphs. Take a look, see what you'd like.