I was just exploring this the other day. There are so so many ...

http://lumtest.com/myip http://lumtest.com/myip.json http://ip.oxylabs.io http://ip-api.com http://api.ipify.org http://ipinfo.io http://icanhazip.com http://ipaddressm.com https://api.myip.com http://httpbin.org/ip http://bot.whatismyipaddress.com https://ipapi.co/json http://ifconfig.io http://ifconfig.pro http://ipfinfo.fun http://checkip.amazonaws.com http://eth0.me http://ident.me http://ipaddress.sh http://ipfree.info https://iphorse.com/json http://myexternalip.com/json http://wgetip.com http://ip.jsontest.com http://thisip.com http://l2.io/ip https://wtfismyip.com/json http://ipecho.net/plain http://ipgrab.io http://whatismyip.akamai.com http://curlmyip.net http://myip.ipip.net http://checkip.dyndns.com http://ip.anysrc.net https://ip.seeip.org http://ipcalf.com

I guess you can count on the proxy networks, the big companies, and the aged ones. Otherwise take your pick based on what's memorable or short.