Answer
Phenylalanine is a stronger acid than Tryptophan.
Work Step by Step
An acid with a small $pK_{a}$ value or a large $K_{a}$ value is a strong acid. A weaker acid, in comparison, has a smaller $K_{a}$ value or a larger $pK_{a}$ value.
$pK_{a}$ of Phenylalanine = 1.83
$pK_{a}$ of Tryptophan = 2.83
smaller value of $pK_{a}$ = 1.83 (for Phenylalanine) and hence it is stronger of the two.
