The best replacement for pi is tau.

The best alternatives to pi are rated as following:
★★★★★  tau
★★★★  phi
★★★  e
★★★  pie
★★★  arduino
★★★  sigma bonds
★★  ph
★★  pid
★  pi
★  ps modifier
★  vlf
★  pka
★  beaglebone
★  npi
★  22/7
★  pie which came first
★  alpha kappa psi
★  3.14
  7 aad
  pi e
  tao
  beaglebone black
  pid controller
  vlf detector
  nnrti
  ci
  npv
  non pi
  psht
  piti
  1080 pi
  golden ratio
  vlf metal detectors
  n pi
  sun quan

Your alternative for pi is not listed? Suggest!
Speaking about pi, one probably shouldn't forget about calculator.

Other search terms related to pi and their ratings are:
★★★★★ calculator
★★★ circle
★★ xbmc
★★ keyboard and mouse
★ remote control
★ arduino
You have in mind a companion for pi and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of pi in all years. Click on graph for more.
pi
tau
phi