Here are some contenders, along with their silent letters:
* "Gnat" - The "g" is silent.
* "Knight" - The "k" and "gh" are silent.
* "Wrought" - The "w" and "gh" are silent.
* "Psalm" - The "p" and "s" are silent.
* "Often" - The "t" is silent.
Ultimately, the "most silent letters" is subjective.