The ones that I'm really not looking forward to are the silvershard mines perfection and silvershard "be at all 5 cap spots when a cart caps" achieves. After those I'll be done with Khan other than the grinding to 100 wins in a couple of them.
Battlemaster was easy for me since I've been playing since Vanilla - Resilient Victory I had to make a premade for, but none of the others were problematic.
Time consuming: The Insane
Lots of Time and Luck: All of the PVP titles from BGs (though they nerfed some achievments)
Extremely Difficult: Ra'Den Heroic, being waaay at the top of the PVP totem pole. Anything that requires you to be in the top .1% of all players is obviously going to be hard.
Hardest: I'd say any arena ones, even arena master I'd say is hard, if you do it the right way.
Longest: Bloodthirsty or the Insane
The achi where you obtain a mount.. jeeeez
Scarab lord in the old days was one per server, that is pretty rare.
I'd have probably said either Death's Demise or Grand Crusader (from Alone in the Dark and Tribute to Immortality Realm Firsts, respectively) before ICC.
Current ones are Storm's End and Tyrannical Gladiator title.
I'm probably forgetting something but I think the rarest titles are these in order:
the Immortal/the Undying
High Warlord/Grand Marshal
Hero of the Horde
I have a few good ones but I usually run with a title that fits the classes lore. Ex: druid with Guardian of Cenarius. Pally w Crusader