At what composition is Maximum deviation from Raoult's law observed?
There's no definitive answer to your question. You might want to look up azeotropic mixtures.
Essentially, to make an azeotrope, you need a set of liquids with really high deviation from &quot;ideal&quot; behaviour, and at the composition where the deviation is maximum, the slope is zero. At that composition, there isn't much variation in vapor pressure as you vary the mole fractions, and so both components of the liquid boil equally, as stated here:

An azeotrope or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling point mixtures.

Now, if you look at a few examples of azeotropes, it becomes clear that the maximum deviation is not achieved at the same composition ratio:

Sulfuric acid (98.3%) / water, boils at 338 °C
||Nitric acid (68%) / water, boils at 120.2 °C at 1 atm||
Perchloric acid (71.6%) / water, boils at 203 °C||
Hydrofluoric acid (35.6%) / water, boils at 111.35 °C

References: Azeotropes, wikipedia