This is a trick. Let's pick a font that is symmetrical vertically,
say font "Agency FB" (hope your computer has that font):

I6, 06, 68, 88,  ?, 98  

If we turn that upside down, or stand on our heads, we'll see:

86 '¿ '88 '89 '90 '9I

and we see that 87 goes where the upside down question mark is:
86 '87 '88 '89 '90 '9I

Now when we turn that right-side up again we have:

I6, 06, 68, 88,  L8, 98  

So the answer is: L8 ! LOL :)  

Edwin