The best replacement for rasengan is chidori.

The best alternatives to rasengan are rated as following:
★★★★★  chidori
★★★★★  kamehameha
★★★★  rasengan
★★  sasuke chidori
★★  raikiri
★★  lightning blade
★  getsuga tenshou
★  chidori gif
★  lightning cutter
★  chidori youtube
★  rotation
★  chidori episode
★  spirit bomb
★  rasen shuriken
★  kakashi rasengan
★  cidori
  chidori video
  chidori stronger
  hadouken
  sharingan
  rasengan kakashi

Your alternative for rasengan is not listed? Suggest!
Speaking about rasengan, one probably shouldn't forget about shadow clones.

Other search terms related to rasengan and their ratings are:
★★★★★ shadow clones
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