The best replacement for proton is electron.

The best alternatives to proton are rated as following:
★★★★★  electron
★★★★★  neutron
★★★★  perodua
★★★  photon
★★  imrt
★★  electron mass
★  an electron
★  positron
★  neutron bigger
★  neutron heavier
★  electron heavier
★  electron charge
★  hydrogen ion
★  neutron mass
★  perodua better
  neutron size
  naza
  cyberknife
  electron size

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Speaking about proton, one probably shouldn't forget about mahathir.

Other search terms related to proton and their ratings are:
★★★★★ mahathir
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