The best replacement for roman numerals is arabic.

The best alternatives to roman numerals are rated as following:
★★★★★  arabic
★★★  arabic numerals
★★  numbers
★  arabic numbers
★  greek numerals
★  roman numerals
  regular numbers

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Speaking about roman numerals, one probably shouldn't forget about ionic compounds.

Other search terms related to roman numerals and their ratings are:
★★★★★ ionic compounds
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roman numerals
arabic
arabic numerals