What is the greatest act of worship that erases sins, and is it the saying of "Lā ilāha illā Allāh" (There is no god but Allah), and does "And whoever fears Allah - He will make for him a way out" apply to a sinner who has repented and turned to Allah?

Sins are expiated by sincere repentance (Tawbah Nasuh), pure monotheism (Tawhid Khālis), effacing good deeds (Hasanaat Mahiya), and expiating calamities (Masā'ib Mukaffirah). The phrase "Lā ilāha illallāh" (There is no deity worthy of worship except Allah) is considered the greatest of good deeds. If a servant realizes the word of Tawhid with their heart, tongue, and limbs, all their sins are forgiven, and it prevents them from entering Hellfire. The greater the Tawhid, the more complete Allah's forgiveness. Whoever fears Allah by abandoning prohibitions and performing enjoined acts falls under this, even if they sin and then repent, for the sincere penitent has their bad deeds expiated and replaced with good deeds.