Titan was the first fight that really got me sitting up and paying attention to mechanics. While I agree that some of the 'hard' fights actually diminish after you acquire the knowledge about the mechanics (completely normal), most of what I've seen post-Titan has been a step above the easy-mode mechanics that you find in most MMOs over the last 5 or so years. I also love that the mechanics simply are what they are, and there is no "do watered down version -> normal version -> hard version" system, obtaining gear for the 'next level' at each step so it is easier to complete following.
That said, I consider myself pretty cluey and catch onto mechanics pretty quickly, which I wish I could say the same for some players who fail 'simple/predictable' mechanics over and over; Had a group wipe on Tera 3 times because they didn't clear their doom, despite it giving you 12 whole seconds to run to the damn glowing square, and that the 2 prior times they'd died for the exact same reason...
Edit: Oh, except The Praetorian and Carpus. The Ultima Weapon fight was cool, but overall I found them really straight forward. Maybe it was getting used to dodging stuff for 40+ levels, or maybe they toned it down because it was story content, I have no idea.