The most-used gems for Unholy Death Knight in The War Within, plus which slots to enchant — aggregated from real Mythic raid and Mythic+ logs on WarcraftLogs.
These are the slots the top Unholy Death Knight players enchant (with how many do). Don't leave them empty — missing enchants are the most common, cheapest performance gap. Open the Unholy BiS page for the exact enchant per slot, or paste your character into the analyzer to see what you're missing.