The best replacement for d20 is gurps.

The best alternatives to d20 are rated as following:
★★★★★  gurps
★★★★  d6
★★★★  d30
★★★  3d6
★★★  nikon d70
★★★  aw110
★★★  aw100
★★  d10
★★  d6 isaac
★★  xti
★★  tg 2
★  tx20
★  ts4
★  tg-1
★  nikon aw100
★  ft4
★  olympus tg-820
★  d100
★  2d10
★  panasonic ts4
  olympus tg2
  canon d30
  ts3
  sony tx20
  tg-820
  olympus tg 2
  d20
  ts20

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d20
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