Dear Ioannins,

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.

Good luck

Aly R Abdel-Moemin

Appreciate for your help, Mr Aly R Abdel-Moemin 

Gallic acid is often used.

thank you both for your help!