This study aims at proposing a modified 7-category fuzzy SERVQUAL to measure service quality for the banking sector. We have added new categories to the conventional SERVQUAL to account for the impacts of the bank’s Corporate Social Responsibility (CSR) initiatives, and e-banking. A questionnaire was distributed among the banks’ customers to measure the criteria’s weights, perceived performance and expected performance. We use the criteria weights, perceived performance and expected performance to test the proposed index for three of the largest Iran’s banks, namely: Bank Melli Iran, Bank Mellat and Bank Saderat Iran which henceforth will be referred to as banks A, B, C throughout the study. A simple Decision Support System (DSS) was devised to determine the most successful bank with regard to CS using Gap model. The result of this study showed that Bank B has the lowest weighted performance expectation gap (-0.051) among the three banks. It also has the highest average performance score in all dimensions and rank the best on 22 out of 30 criteria. Finally, sensitivity analyses are conducted to assess the impact of criteria weights and fuzzy numbers on the evaluation/ranking process. The resultant index helps managers in micro, meso and macro levels to detect the frailty zones and atone for the dissatisfying functions.