Make the denominator equal for each. I'm going with the denominator 30.
For 1/2: Multiply the denominator and numerator by 15 to get...

For 1/5: Multiply the denominator and numerator by 6 to get...

For 1/6: Multiply the denominator and numerator by 5 to get...

Since now the denominators for all are the same, we can compare it properly seeing that 1/2 is the largest, then 1/5, and then 1/6 is the smallest.