Okay, so far :
Dragon Trail Seaside - diamond - fairly easy, but if you start with this track you will find it hard to drive the car at the beginning
Lago Maggiore - diamond (by far the hardest for me, took me really a lot of time, ~5-6 hours maybe )
Monza - diamond - this one is probably the easiest
Interlagos - diamond - that's my strongest track, I was expecting to be done with it in a matter of minutes but took me an hour, hour and a half probably to get it right
Brands Hatch - diamond - was expecting to be really really hard especially, when i watched some really good players on it, but took me probably an hour, hour and a half, but again most of the people didn't have problems with Lago Maggiore while I was beating my head there.
What's left (haven't tried them, usually I stick to a track until I get a diamond)
Willow Springs - haven't tried it yet, but I see lots of people consider it as the hardest - I think I'm fairly good at it, but we will see.
Suzuka - I like the track, hopefully will not have lots of problems there
Bathurst - this one I almost haven't played on, so don't know what to expect here
Nurburgring GP - I'm really bad at this track, so expect some time to get things right
Nordschleife - don't know what to expect here, really
I find this DLC a really good training for my lines - so far I've improved a lot on all the tracks I've diamond already, let's see if I'm going to be able to diamond the rest ...