For me personally my most prestigious title is Hand of A'dal. However the most prestigious would have to be Scarab Lord, but as I didn't even play back then, ah well. However people who are saying the realm first achieves from Wotlk are prestigious it really depends on the server, hell there's a few realms with only 1 raiding guild on them. So it really depends on the guilds whoever got it was competing against, there has been a history of high end guilds migrating to low pop realms for latency issues etc, so there was no challenge if there is no guilds around the same standard as those who got it. When it comes to the immortal achieves etc... really? do you really think it was hard to go back and do content 1-2 tiers lower while in bis ICC gear? not prestigious at all imo.
What I think they should do is: Apply Certain titles to those who get Cutting edge achieves, and a different title to the hc achieve. That way they can reward higher end raiders while still giving a possible incentive to players to go back and experience previous tiers.