The best replacement for nightingale armor is dark brotherhood.

The best alternatives to nightingale armor are rated as following:
★★★★★  dark brotherhood
★★★★  ancient shrouded armor
★★★★  thieves guild
★★★  guild master
★★★  ebony mail
★★  guild master armor
★★  dragonscale
★  shrouded armor
  ebony armor

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Speaking about nightingale armor, one probably shouldn't forget about ebony mail.

Other search terms related to nightingale armor and their ratings are:
★★★★★ ebony mail
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nightingale armor
dark brotherhood
ancient shrouded armor