Back in my day, we killed bosses 400 times a day...uphill both ways...in the snow...and we fucking liked it. In related news, I hear that uninstalling WoW completely from your hard drive and then reinstalling it resets your RNG numbers. -septor
Are we talking about ones that are still attainable? I'd say the HK or Battlemaster (possibly Conqueror if you missed the Warsong / Arathi rep from classic).
Unattainable ones like classic High Warlord, Scarab Lord done properly and some of the ones from Ulduar were hard as nails too. I struggle to really think of any particularly prestigious PVE titles bar realm firsts these days - especially after 'the Insane' has been repeatedly made easier - I wish they'd have retired it and made a new version for people to work on, heroic kills take a few hours over a few months to get, PVP ones really need you to put in the time and effort unless you're doing that stupid AV hk farming.
If were talking currently available, Herald of the Titans is a serious grind. Getting 10 people to lvl to 80, stop xp, grind gear and then fight Algalon as he was intended to be fought. Still not the easiest of things. I'm contemplating trying for it myself in the hopes that it would be shared through all toons come MoP. Hardest part of course is finding those 9 other people willing to do the grind.
Hardest Title for my guess would be Firelord, if you don't buy it.
Firelord is a joke, any of the PVE titles are sadly easy to get and aren't very prestigious at all. I wore Dragonslayer for awhile during Firelands, but when everyone started to get it during DS I stopped wearing it. Currently I wear The Undying as my PVE title of choice, or Conqueror of Ulduuar. Both of which are unobtainable. The prestigious pve titles are no longer obtainable. The ones that are mean nothing, even Savior at this point.
I wouldn't even say the RBG titles are worth showing off, 7 months into the season it means next to nothing to most people. Maybe on your realm its something, but on mine you're just another brick in the wall with Grand Marshal. Duelist would be okay to sport, Gladiator would be ideal. R1 is not something available to everyone obviously but thats the most prestigious I can think of.
When MoP comes out, getting pve titles and such in a time oriented fashion will be cool to sport. That kinda thing happens with the PvE titles nowadays. Theyre worth showing off for a couple weeks after the raid content is out, then it becomes completely irrelevant and no one cares because everyone has it.