The best replacement for intake manifold is headers.

The best alternatives to intake manifold are rated as following:
★★★★★  headers
★★★★  exhaust manifold
★★★★  head gasket
★★  skunk2
★  rbc
★  blox
★  ls1
★  plenum
★  edelbrock
★  type r intake manifold
★  ls6
  edelbrock intake manifold
  p30
  skunk2 intake manifold
  header
  d16z6 intake manifold
  rbc intake manifold

Your alternative for intake manifold is not listed? Suggest!
Speaking about intake manifold, one probably shouldn't forget about egr.

Other search terms related to intake manifold and their ratings are:
★★★★★ egr
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intake manifold
headers
exhaust manifold