My impression is that there are no "hardest tests" as such, because the difficulty depends on the users driving style.
Many users seem to struggle with B-5 (the Tsukuba test), which I golded in the second attempt. I do however struggle with B-6 (Dunlop corner @Nürburgring), which I've clocked 500 attempts at so far with at least 80 times missing it by less than 0.050 seconds. I'm sure it was easy as pie for most others, but I'm stuck at it.
GT5 has however some almost physically impossible ones (at least with the DS3). The Alfa Competizione thing on Eiger short is a joke and a very bad one at that. Drives me absolutely insane and since the 1.06 nerf... er, I mean patch... the last NASCRAP challenge is nigh-on impossible without resorting to dirty tactics, since it is impossible to turn off that anti-skid malarkey, which a NASCRAP canal boat doesn't have in the first place.
Generally, I'm rather disappointed that half of GT5 is now just useless time-trialling, while A-Spec is reduced to a handful of races carbon-copied from GT4.
What doesn't help either is that many of the "example videos" in the license tests are completely useless, since they didn't make the gold time themselves. Now that's as useful as a one-legged bloke at an arse-kicking contest.
I'm going with the tactics that made me gold all licenses in GT2. Try, try, try and if it still isn't working - retry. I eventually made it after 14 months and an aggregated count of about 20.000-30.000 attempts on the 60 tests.
Problem is, now I'm old, bald and have something called "a life", so I probably won't make it in GT5
