Nightmare is really cool, I mained him alongside Cervantes back in 5. I haven’t seen a nightmare online yet. Right now I just use Cervantes I feel really comfortable using him.
Groh is up there, but I run into at least 10 Nightmares for every Groh.
Aside from Nightmare, I'd say the most common characters I run into are Talim, Groh and Mitsurugi.
Kilik, Taki and Yoshimitsu would be a tier after that.