First off Meessaview, there is no "perfect lens" for rainbows - the composition will always dictate the lens - depending if you want to capture the entire rainbow or go tight and shoot a partial rainbow - that will always be your choice. Moreover, you can only capture what is there. The lens will not make the rainbow any brighter or more saturated. A polarizer will certainly help as it will saturate the rainbow or completely eliminate it. You can add some contrast in post and try playing with Vibrance and/or DeHaze.