Before achievements were account-wide, and leaving aside obvious ringers like Light of Dawn/Bane of the Fallen King before Mists, I would have nominated Loremaster or Seeker as the most reassuring titles to see on a player regardless of gear quality. I can't say exactly what link exists between the persistence needed to get these titles and overall competence, but I have yet to see a Loremaster or Seeker player do something screw-the-pooch crazy in a PUG. These days, it's tougher to guarantee that a player sporting a title is doing so on the character that originally got it, so maybe The Undaunted is probably the single best outward indicator of competence.
Having written all this, I've also had the pleasure of knowing a world-class hunter who never bothered using titles, and we have to acknowledge that a generic, funny (e.g., "Chef Boyardee"), or absent title means nothing at all. But if you had to pick one that said, "I will not get you killed in this PUG," which would it be?