Oftentimes, when we do organic chemistry problems we must determine what kind of reaction we have. For the SN1, SN2, E1 and E2, our substrate will have a leaving group that will leave in the process of the reaction. So how can we identify leaving groups and tell which ones are better?
Well, a leaving group is considered to be a good leaving group if, after it leaves, it is stable on its own. Therefore leaving groups that would have resonance are stable as well as leaving groups who are large.
Most often encountered leaving groups are halogens (Group17). The larger the atomic radius the better of a leaving group halogen is and therefore, I- > Br-> Cl-
Thus, according to the David Klein book here is the list of best to worse leaving groups.