If I have an area of a page with different options for filtering the search results (unordered lists with links, checkboxes, selects, etc.). What html5 tag should be used to wrap that filters? A "section" tag, a "nav" tag or something else?

<div id="search-filters"> <!-- This should be a div, a nav, a section? -->
    <h3>Price</h3>
    <ul>
        <li>less than 100</li>
        <li>100 - 200</li>
        <li>more than 200</li>
    </ul>

    <h3>Brand</h3>
    <ul>
        <li>Brand A</li>
        <li>Brand B</li>
        <li>Brand C</li>
    </ul>

    ...
</div>
<section id="search_results">
    ...
</section>

You could use the header element:

The header element represents a group of introductory or navigational aids.

Note that the header needs to be in the sectioning element (in your case, section) of the search results:

<section id="search_results">

  <h1>Search results</h1>

  <header id="search-filters">
    <!-- your filters -->
  </header>

  <article>
    <!-- search result 1 -->
  </article>

  <article>
    <!-- search result 2 -->
  </article>

</section>

You could include the h1 in the header too, if you like. If you then need a styling hook for the filters, you could use a wrapper div.

If your filters are rather complex, e.g. if you offer many filters, probably with subheadings, you could use a section element inside of the header:

<section id="search_results">

  <h1>Search results</h1>

  <header id="search-filters">
    <section>
      <h2>Filter the results</h2>
      <!-- your filters -->
    </section>
  </header>

  <article>
    <!-- search result 1 -->
  </article>

  <article>
    <!-- search result 2 -->
  </article>

</section>