The best replacement for shadow priest is warlock.

The best alternatives to shadow priest are rated as following:
★★★★★  warlock
★★★★  mage
★★  holy priest
★  frost mage
★  mage 5.4
★  balance druid
★  warlock 5.4
★  elemental shaman
★  affliction warlock
★  warlock pvp
★  destruction warlock
  discipline priest
  ret paladin
  druid
  lock
  warlock cataclysm
  enhancement shaman
  warlock mop
  death knight
  disc priest
  feral druid
  fire mage

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Speaking about shadow priest, one probably shouldn't forget about haste.

Other search terms related to shadow priest and their ratings are:
★★★★★ haste
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