It's obviously Scarab Lord, that is, if you're not counting early season arena titles.
It's largely a personal thing. On some realms scarab lord got gutted by bugged out realms, along with the whole fiasco in wrath of hundreds of people getting the 24 hour window on the last new realms. It is a consistently rare title, but currently is more common than plenty of titles (either due to not being used as OP asks or people having quit), such as mentioned hand of ad'al, magic seeker, so on because people who were raiding properly in tbc were rare these days, and people who got the realm firsts 4 years ago naturally are much harder to find.
Originally Posted by Endus
which is kind of like saying "of COURSE you can't see the unicorns, unicorns are invisible, silly."