The best replacement for sb 900 is sb 700.

The best alternatives to sb 900 are rated as following:
★★★★★  sb 700
★★★★  sb 600
★★★★  sb 800
★★★★  910
★★★  sb 910
★★  metz 58
★★  alien bee
★★  800
★★  580ex ii
★  nissin di866
★  canon 580 ex ii
  yongnuo
  nikon sb 600

Your alternative for sb 900 is not listed? Suggest!
Speaking about sb 900, one probably shouldn't forget about d2x.

Other search terms related to sb 900 and their ratings are:
★★★★★ d2x
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sb 900
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