CT access quest is pretty terrible right now because the FATE spawn timers are essentially bugged, and you could spend 4+ hours unlocking it if you're unlucky.
Moogle King is also a pretty long unlock, but the rest are very fast.
Extreme Mode Garuda is easy enough to do with a competent FC, or it will be puggable quite soon once strats get out there. It's all just doing the dance, without the reaction times required by Extreme Titan (who really isn't much harder than Hard, at least relative to the level of content).
I haven't done Ultima yet, but I've been told it's not really a hard fight, but just has a whole lot of things to dodge.
About to do my first HM dungeon. Did Pharos Sidius earlier and that was by far more challenging than AK ever was, so I have high hopes.