If the issues with your current setup are low constrast and looking washed out, then a new lens isn't going to help. A polarizer will help in some cases when there is some blue sky in the picture or a rainbow, but otherwise it won't make much difference either.

What will matter is post processing. Clouds are all quite bright, even the "dark" ones. The camera exposes for the highlights so they don't clip, and the darker tones come out where they come out. For example, if the brightest part of the clouds is exposed to just under saturation and the darkest parts are 1/8 as bright (will look quite dark for a cloud), then the darkest part of your image won't be anywhere near black. This will make the picture look washed out.

The first thing to do in post processing is to set the darkest and lightest parts of the picture to black and white, respectively. That alone will probably make a large difference. Then you can play with non-linear brightness curves and other effects after that.

Since you didn't post a picture, I'll use one of my own to illustrate this.

Here is the way I originally post-processed it so that all parts of the picture are visible and not clipped:

Just to illustrate the point I ignored the forground and made the clouds span the full dynamic range of the picture:

All I did was map the darkest to lightest area of the clouds to black to white of the picture. That caused the darker forground to be completely black, but this is just to show what you can do with clouds. If I spent a little more time and wanted something like the second picture without the foreground looking patchy, I'd use some non-linear tone mapping so that dark parts fall off rapidly to black. Usually you want the opposite. I don't have the raw file here on this computer, so I'm not going to do that right now, but I think you can see the point I'm trying to make anyway.

Now that you've added pictures to your question I can use them as examples. The first is tilted and looks silly (is really that hard to hold the camera straight?), the second doesn't have much going on, so I'll use the third, even though it has blown highlights in the bright area near top left. Unfortunately those blown highlights will limit what we can do.

Here is your original:

We can immediately see why this looks washed out. The darkest area is (.048, .143, .185). Setting the full dark to light range to map to black to white in the picture yields:

Taking this further, the black level can be set to the darkest area of the clouds ignoring the ground, and adding a little non-linear brightening so that everything except the blown area doesn't look quite so dark:

As you can see, the clouds look consideraly more dramatic than in the original, although I think too dark. Much of the ground is now below black and splotched out. If you really wanted to work on this picture, I'd say to create a mask for the ground so that it can be independently brightened relative to the clouds. I'd also want to brighten up the light parts of the cloud at lower right to make its texture pop more and not look so dingy.

Anyway, the point is that you can't expect these kinds of picture to look good right out of the camera, but the right picture can be made quite dramatic looking with some careful post processing. However, don't blow out the highlights next time. Blown highlight can easily turn into splotches or even have Mach bands around them after post processing that brings out highlight detail in the rest of the picture.