Answer
methyl iodide> methyl chloride> ethyl chloride> isopropyl bromide> neopentyl bromide> tert-butyl iodide
Work Step by Step
Methyl halides react fastest toward SN2 reaction. Methyl iodide reacts faster than methyl chloride because I- is much a better leaving group than Cl- (I- is larger and can let stabilize the negative charge better). After methyl chloride comes ethyl chloride because ethyl chloride is still a primary halide. Next is isopropyl bromide, which is a secondary halide. Neopentyl bromide follows. It is more reactive than tert-butyl iodide despite being bulkier than tert-butyl iodide. This is because neopentyl bromide is still a primary halide whereas tert-butyl iodide is a tertiary halide, which almost doesn't react toward SN2 at all.
