I would say the OP's orginal assumptions are false.
The most important workout depends on what you are training for....same for the second, third, fourth, and so on.
It also depends on your own personal strength and weaknesses.
For me, my most important workout is a 6 mile out-and-back "tempo" that starts at "marathon effort" for the first 3 miles and cuts down from there. This is the same for any distance I am training for. If I can "cut down" signifcantly in the last 3 miles then I know I am in shape.
My second most important workout are mile repeats with .2 rest. There is a park in Louisville that has a large 1.2 mile circle measured evey 1/4 mile. I bust each mile at a hard pace...we'll say 5k effort...then jog the .2 back to the start (2:00-2:30 jog). It's a good test, I'll usually only do 4 then if I'm having a rather good day I'll bust open 4x400. At first I'll be slow and slower the last one. When I'm in shape I'll hit every one even and fast.
My third is a longish run (13-18+) that starts relatively easy and finishes much harder in the last 30-45 minutes. This run just "feels good".
Find out what makes you a better runner, find out what makes you "get into shape" and "feels good". And repeat that. Those are YOUR most important workouts.
Alan