The best replacement for udid is imei.

The best alternatives to udid are rated as following:
★★★★★  imei
★★★★★  iccid
★★★★  idfa
★★★  uuid iphone
★★  mac address
★★  meid
★  device token
★  uuid
★  serial number

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

Other search terms related to udid and their ratings are:

★★★★★ ios
You have in mind a companion for udid and it is not yet here? Propose!

Google Trends

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