Try this one. Place the four fingers of your left hand on the top (thinnest) string at position 5 chromatically, i.e. index on fret 5, middle on fret 6, ring on fret 7 and pinky on fret 8.

Then, whilst leaving all other fingers on the fretboard, lift your index finger off the string and move it over to the second string. Do some pull-offs or trills between fret five and the open string. Move to the third string and do the same and repeat for each string until you are down to the bottom string. You will see that your index finger is stretching further with each string.

Return your index finger to position five and then do the same with your middle finger, making sure that the other fingers remain seated at the fretboard.

Which finger is most difficult? I find it's the ring finger and I'd guess that would be the same for mose people.