To be able to make the highest possible solution you must find the position to put the highest number and then for the next and so on until the lowest number cube can be in the spot where the least faces can be exposed.
6: would be on the top space on the staircase because that is the spot where most faces can be exposed so the are more of the highest number to make an even larger number
5: AS obviously known the 5 is the next highest number so that means you need to find the spot where the next most faces will be exposed which is the spot diagonal to the 6 on the staircase.
4: the next spot is a the front at the bottom of the staircase on the first step
3/2: it doesn't really matter where you put these two out of the two possible positions that is the next amount to be exposed because they're the same amount of faces to be exposed it is either the second level right at the back or the spot underneath the other one.
1: final one is the middle on the bottom row and that is where the one is supposed to go because that is the spot where the least exposed faces and 1 is the lowest number possible.
the answer is:
6: there is five sides showing 5x6=30 30
+ 20
5: there is four sides showing 5x4=20 + 16
+ 09
4: there is four sides showing 4x4=16 + 06
+ 01
3: there is three sides showing 3x3=9 = 81
2: there is three sides showing 2x3=6
1: there is two sides showing 1x2=2