With pinggy.io you can reach a Raspberry Pi over SSH from anywhere with just a single command, without port forwarding or a static IP. Replace the port 22 with the port where sshd is listening.
Enable SSH on the Pi first with sudo raspi-config (Interface Options → SSH), then run the tunnel command on the Pi itself. This is a TCP tunnel, so Pinggy gives you a host and port such as tcp://example.a.pinggy.link:11223.
From any other machine, connect with the host and port Pinggy printed:
ssh -p 11223 pi@example.a.pinggy.link
To keep the tunnel up across reboots, see running a tunnel on startup.