The best replacement for imei is esn.

The best alternatives to imei are rated as following:
★★★★★  esn
★★★★  serial number
★★  imsi
★★  meid
★★  udid
★  imeisv
★  iccid
★  android id
★  mac address
★  tac
★  sim number
  meid number

Your alternative for imei is not listed? Suggest!
Speaking about imei, one probably shouldn't forget about sim card.

Other search terms related to imei and their ratings are:
★★★★★ sim card
★★★ phone
You have in mind a companion for imei and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of imei in all years. Click on graph for more.
imei
esn
serial number