When I booked my fastpasses 60+ days out last week - Frozen was still the hardest slot to get. I grabbed one - and they were only available from dinnertime and later (worked for me as that was what I wanted) , went back 30 minutes later to try to move it and there was literally one time slot available for a party of 4. Granted I was going to Epcot on the second day of my vacation, so many others with longer vacations had already raided Frozen for the day.
7DMT next in line in difficulty.
Pandora wasn't hard to get.
Didn't try for ROL - we are doing a Tusker House breakfast dining package that includes tickets for seats.
Toy Story Mania was medium difficulty for me only because I was looking for a particular time slot, and had very little flexibility with it.
Soarin' - didn't try as it is part of our early morning EMH plan one day. Also, when we were there on MLK weekend standby lines were not unmanageable if you hit it at the right time.
I think a combination of the day number of your visit and the difficulty of the fastpass works best. If someone has a 7-day vacation that starts 6-days before you, they have already had first dibs at the fastpasses for your first day. The easiest fastpasses to book are the ones at the end of your vacation.