What do you mean by "reactivity". Moreover, the esterification is carried out in the presence of catalysts. Also, phthalic acid is not used, but its anhydride. Unfortunately, there is no definite answer to your question.

Simply, multiple carboxylic groups, requires more time for the reaction completion. It is import to remove the water forming in the reaction mixture to assure the completion. Other than that the esterification of di carboxylic acid is indeed achievable.

I suppose the substitution pattern on the phthalic acid isomers used could be the reason for the reactivity. Phthalic acid has the dicarboxylic acid on the ortho position. It is generally regarded the most reactive and most toxic. Isophthalic acid has on the meta position while terephthalic has it on the para position and is regarded less toxic and less reactive.

In esterification, the rate of the process is influenced not only by the organic acid, but also by the alcohol and catalyst used. It will be different with the use of methanol and different with the alcohol isomers, where steric effects may play a role. Moreover, as I wrote, no one esterifies phthalic acid, only its anhydride. It is also worth knowing that in the case of anhydride, a monoester is formed almost immediately. Just mix the substrate and heat it to the desired temperature. Therefore, it is not possible to answer the question without carrying out the appropriate experiments.