Iodide snaps off carbon in a flash, while fluoride clings on. Why?
A good leaving group is a weak base. It stays stable holding the electron pair it takes away.
Size is key. Iodide's huge cloud spreads the negative charge thin, so it is calm alone.
Check the conjugate acid. HI is a strong acid; HF is weak. Strong acid means weak base.
So the ranking is iodide, bromide, chloride, then fluoride: weaker base, better leaving group.