The best replacement for 18650 is aa.

The best alternatives to 18650 are rated as following:
★★★★★  aa
★★★★★  18350
★★★★  cr123
★★★★  cr123a
★★★★  18500
★★★  2 cr123a
★★  14500
★★  26650 battery
★★  2 cr123
★★  18650b
★★  18490
★★  17670
★★  26650
★  aa battery
★  3 aaa
★  16340
★  cr123 battery
★  cr123 batteries
★  rcr123
★  18500 batteries
★  26650 batteries
★  two cr123
★  d cell
  18350 battery
  16650
  123a
  14500 battery
  cr123a batteries
  3 x aaa
  stacked 18350
  3aaa
  18700
  two 18350

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Speaking about 18650, one probably shouldn't forget about kick.

Other search terms related to 18650 and their ratings are:
★★★★★ kick
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Google Trends

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18650
aa
18350