Rainbows appear in different saturation levels, based on the contrast of the seen the weather and angle of the sun. Capturing raw will give a lot more data and range to work with, further enhancing the rainbow and adjusting to seen to make a rainbow pop. Photoshop is nothing but a tool to do so, you could also use Lightroom, ACR, Capture One or any other post processing software.
If you want to get an out of the camera image to use, then I suggest you try to create a picture profile that would give you a desirable JPEG, assuming you want to avoid post processing. But then you would lose on the potential of an. image if you don't shoot raw.
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.