I worked on my DeathKnight’s home city factions since he was close to exalted on all of them.
He now goes by the title Ambassador (from earning [Ambassador of the Alliance] today). Like my Priest before him, all he needs to do is earn Exalted status with the Argent Crusade and he’ll have his Crusader title/achievement.
I think Ambassador is a good title for him — he’ probably stick with that even after earning Crusader. I mainly want the achievement complete so he can continue to do all available Argent Daily quests once patch 3.2 hits.