The best replacement for cactus v5 is pocket wizard.

The best alternatives to cactus v5 are rated as following:
★★★★★  pocket wizard
★★★★  yongnuo rf-603
★★★  phottix strato ii
★★★  phottix strato
★★  rf-603
★★  cybersync
★  pixel king

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Speaking about cactus v5, one probably shouldn't forget about alien bees.

Other search terms related to cactus v5 and their ratings are:
★★★★★ alien bees
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cactus v5
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yongnuo rf-603