The most commonly used Tailwind CSS classes are:

* **Flexbox:** `flex`, `flex-row`, `flex-col`, `justify-center`, `justify-between`, `items-center`, `items-start`
* **Spacing:** `p-{n}`, `px-{n}`, `py-{n}`, `pt-{n}`, `pr-{n}`, `pb-{n}`, `pl-{n}`
* **Typography:** `text-{size}`, `font-{weight}`, `text-{color}`, `leading-{size}`
* **Colors:** `bg-{color}`, `border-{color}`, `text-{color}`
* **Responsive:** `sm:{class}`, `md:{class}`, `lg:{class}`, `xl:{class}`, `2xl:{class}`

These classes can be used to create a wide variety of layouts and designs, and they are all responsive by default, meaning that they will adapt to different screen sizes and devices.

Here are some examples of how to use these classes:

```html
<!-- Flexbox -->
<div class="flex flex-row justify-center items-center">
  <button class="bg-blue-500 hover:bg-blue-700 text-white font-bold py-2 px-4 rounded">Click me!</button>
</div>

<!-- Spacing -->
<div class="p-4 bg-gray-100">
  This is a box with 4px of padding on all sides.
</div>

<!-- Typography -->
<h1 class="text-3xl font-bold text-gray-800 leading-tight">This is a heading</h1>

<!-- Colors -->
<button class="bg-green-500 hover:bg-green-700 text-white font-bold py-2 px-4 rounded">Click me!</button>

<!-- Responsive -->
<div class="md:flex md:justify-center md:items-center">
  This is a box that will be flexed on medium screens and larger.
</div>
```

These are just a few examples, of course. There are many other Tailwind CSS classes that you can use to style your web pages and applications. For more information, please see the Tailwind CSS documentation: https://v2.tailwindcss.com/docs.
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