Cloudflare: https://ipv4.icanhazip.com + https://ipv6.icanhazip.com
Akamai: https://ipv4.whatismyip.akamai.com + https://ipv6.whatismyip.akamai.com
The problem with https://checkip.amazonaws.com is that it neither supports IPv6, nor claims that it will remain IPv4-only, so it's not clear what you'll get in the future.