If the intensities of the two interfering beams in Young's double-slit experiment are I1 and I2, then the contrast between the maximum and minimum intensities are good when |I1 - I2| is large, |I1 - I2| is small, either I1 or I2 is zero, or I1 = I2?

Contrast is high when there will be maximum difference between the maximum and minimum intensities in the given fringes. If intensities are same then minimum intensity fringe is zero and it becomes very easy to differentiate between the maximum intense fringe with least one. If intensities are not same then minimum intensity fringe will have some energy and contrast will be lower with respect to maximum intensified fringe.