Hello,
Which from tunneling protocol supported by Mikrotik Routeros is the most difficult to block using firewalls?
Gre, openvpn, pptp, sstp, l2tp/ipsec, gre, vpls?
SSTP, since it works on TCP 443, same as HTTPS.
Are you kidding? If you tie your ability to detect a protocol merely to a port then you are gravely mistaken.
If you block TCP 443, then you block every internet website that uses HTTPS.
Also, every tunneling protocol has a default port that it uses, if you dont like that, change it.
Also, you will find that most restrictive networks block ALL protocols and ports, other then well known ones.