The best replacement for death knight is warrior.

The best alternatives to death knight are rated as following:
★★★★★  warrior
★★★★★  paladin
★★★  mage
★★★  warlock
★★  monk
★★  druid
★★  paladin tank
★★  shaman
★  warrior pvp
  hunter pvp
  warlock 5.4
  warrior mop
  shadow priest
  pally
  priest
  warrior wow
  frost mage
  dread lord
  death knight

Your alternative for death knight is not listed? Suggest!
Speaking about death knight, one probably shouldn't forget about dual wield.

Other search terms related to death knight and their ratings are:
★★★★★ dual wield
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Google Trends

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death knight
warrior
paladin