In this study, 143 different working fluids have been analyzed for Rankine and Brayton cycles in terms of performance characteristics such as power, thermal and exergy efficiency, and EFECWOD. The selection of working fluid is a significant consideration in the design of both these cycles, as it can importantly affect the performance and efficiency of the system. As of late, there has been growing interest in investigating the effects of various working fluids on the performance characteristics of these cycles. This article aims to determine the ten best among different working fluids according to the determined criteria using the Technique for Order of Preference by Similarity to Ideal Solution, which is a multi-criteria decision method. In the decision-making process, the importance scale of the analytical hierarchy process was used to determine the weight values of the criteria to be used in the TOPSIS analysis to obtain more accurate results. Artificial Neural Network method is employed to identify the optimal working fluid as well. As a conclusion of this thermodynamic analysis of the performance characteristics for Rankine and Brayton cycles using various working fluids, the Rankine cycle achieved the maximum power of 12,277 kW, the maximum efficiency of 93 %, and the maximum EFECWOD value of 9962 kJ/m3 with hydrogen, helium, and dimethylcarbonate as the respective working fluids. Furthermore, hydrogen exhibits the highest power output of 2493 kW in the Brayton cycle. Nitrogen demonstrates the highest efficiency at 44 %, while R141b achieves the highest exergy efficiency at 98 %. Lastly, the fluid with the highest EFECWOD value is R13, with a measurement of 4932 kJ/m3.