Share a powerful dhikr for seeking complete forgiveness this Ramadan.

Asalamu alaikum, brothers and sisters. With Ramadan upon us, I'm really hoping to earn Allah's mercy and have all my past mistakes wiped clean. Does anyone know a specific dhikr or dua that’s especially effective for seeking total forgiveness? I want to make the most of this blessed month and truly purify my heart. JazakAllah khair in advance for your guidance!