based on time/effort put in. Which were the hardest and/or the one's you're most proud of? There is no right wrong since it's about personal experience.
In order to list it, you must have displayed it fairly recently on at least one toon otherwise the achievement must not be all that special to you.
1. Delver of the Vaults - Brand new tier, lots of bosses killed in blues and hundreds of attempts for some of the bosses).
2. Crusader & Chef (tie) - Listing them because they were the longest solo titles for me. Don't have Salty yet.
3. Kingslayer - This in honor of how rough ICC was pre-buff, completed with a 15% buff (I think).
Honorable mentions: the Exalted (simply from it taking so many reps across expacs) & the Bloodthirsty (only real pvp title I have lol)
Have fun reminiscing!!