The best replacement for sigma is tamron.

The best alternatives to sigma are rated as following:
★★★★★  tamron
★★  canon
★★  pi bonds
★  canon lens
★  standard deviation
★  nikon
★  mac brushes
★  omega
★  nikon lenses
★  tamron lens
★  tamron 70-200
★  sedona lace
★  canon 50mm 1.4
★  s
★  pi bond
★  canon lenses
★  lean
★  glock
★  nikkor
★  tamron lenses
★  nikon lens
  tamron 17-50
  real techniques
  crown brushes
  s for standard deviation
  real techniques brushes
  tamron for canon
  canon 70-200
  tamron for nikon
  tamron 17-50 2.8
  tqm
  m&p
  cpk
  total quality management
  tamron 24-70
  tamron 18-200
  six sigma
  canon 24-105
  tokina
  nikon 18-200
  lean manufacturing
  canon 35mm 1.4
  6 sigma
  tamron 18 200
  p95
  nikon flash
  tamron 70-300
  bdellium

Your alternative for sigma is not listed? Suggest!
Speaking about sigma, one probably shouldn't forget about kenko.

Other search terms related to sigma and their ratings are:
★★★★★ kenko
★★★ dpmo
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