Honestly, this question feels like it is trying to shortcut/replace a lot of what goes into learning your class. Despite that, I'll list some I like.
Tidy Plates: Threat Plates
VuhDo (click casting Raise Ally as well as watching buffs/debuffs on the raid)
Elkano's Buff Bars
Blood Shield Tracker (Tracks your blood shield and DS heals as well as a rune display)
Icehud (easy watching hp/runic power)
ClcDK - rotation helper
I use plenty of other addons for bags, bar mods, broker bars, etc, but those are the ones I can remember off the top of my head that specifically help with the DK class.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng