Final answer:
The best post-hoc test for equal group sizes is Tukey's HSD test because it compares all pairs of means while maintaining the family-wise error rate and has more power for pairwise comparisons.
Step-by-step explanation:
When group sizes are equal, the best post-hoc test to use is typically Tukey's HSD (Honestly Significant Difference) test. This is because Tukey's HSD is designed to compare all possible pairs of means while controlling the family-wise error rate, which is ideal when groups are of equal size. This test is robust and widely used for comparisons when we have equal sample sizes, and it is generally more powerful than other tests such as Bonferroni, which tends to be more conservative, or Scheffé's test, which is more appropriate when the group sizes are unequal or when we are interested in complex comparisons. Dunnett's test is specifically used when comparing multiple treatments against a single control and therefore is not the best choice for all pairwise comparisons.