For speed runs for me 2 was without a doubt the hardest one. 7 and 9 were both challenging, but mainly because I tried to follow the youtube paths on both rather than go a different way that was slower but easier for me. On 7, for example, I found it easier to kill the guards in the first segment rather than try to run up to the rafters before they were able to take me out. True, speeding through that segment gets a better time, but I was not trying to set any records. Likewise, in 9 in the server room I found it easier to run around the staircase on the left and disarm the first guard, then use that gun to take out the next two guards on the platform, then grab the heavy machine gun from the middle guard and fry the servers. Again, this is slower than many of the youtube routes, but much easier for me.
I agree with the first post's listed 5 time trials. Atrium 1, in particular, is off the charts hard. I found Atrium 2 pretty easy and 3-starred it even though I only needed 2 stars on it by the time I got around to it.
I found the DLC time trials to be more difficult on average than those in the main game. In particular, Flow vexed me. I did eventually get 3 stars on it (offline, alas), but was never able to perform the incredible time-saving Dicey Launch maneuver with any regularity. In 2 hours I only managed to make that jump 3 times. I eventually gave up and went to the left, which is several seconds slower but something I could do much more often than 3 times every 2 hours. Man, that sucked.
Anyway, good luck to those still hacking at the time trials and speed runs. I can at least assure you from my experience that the joy of achievement is worth the frustration of failure. Phew.