If you gonaa to determine total flavonoids quercetin is the best, as you know is more stable compound in the flavonol family also rutin is quercetin the difference between them is a water molecule. Myricetin is another compound theoretically is stronger than quercetin however, the presence of the 3 hydroxyl groups on myricetin B ring 3'4'and 5' make this compounds the weakest as antioxidant and can not be used as standard. Gallic acid may be used as standard but in case of total phenols.