element with the max height from a set of elements

Use .map() and Math.max.

var maxHeight = Math.max.apply(null, $("div.panel").map(function ()
{
    return $(this).height();
}).get());

If that's confusing to read, this might be clearer:

var heights = $("div.panel").map(function ()
    {
        return $(this).height();
    }).get();

maxHeight = Math.max.apply(null, heights);

The html that you posted should use some <br> to actually have divs with different heights. Like this:

<div>
    <div class="panel">
      Line 1<br>
      Line 2
    </div>
    <div class="panel">
      Line 1<br>
      Line 2<br>
      Line 3<br>
      Line 4
    </div>
    <div class="panel">
      Line 1
    </div>
    <div class="panel">
      Line 1<br>
      Line 2
    </div>
</div>

Apart from that, if you want a reference to the div with the max height you can do this:

var highest = null;
var hi = 0;
$(".panel").each(function(){
  var h = $(this).height();
  if(h > hi){
     hi = h;
     highest = $(this);  
  }    
});
//highest now contains the div with the highest so lets highlight it
highest.css("background-color", "red");

If you want to reuse in multiple places:

var maxHeight = function(elems){
    return Math.max.apply(null, elems.map(function ()
    {
        return $(this).height();
    }).get());
}

Then you can use:

maxHeight($("some selector"));

function startListHeight($tag) {
  
    function setHeight(s) {
        var max = 0;
        s.each(function() {
            var h = $(this).height();
            max = Math.max(max, h);
        }).height('').height(max);
    }
  
    $(window).on('ready load resize', setHeight($tag));
}

jQuery(function($) {
    $('#list-lines').each(function() {
        startListHeight($('li', this));
    });
});
#list-lines {
    margin: 0;
    padding: 0;
}

#list-lines li {
    float: left;
    display: table;
    width: 33.3334%;
    list-style-type: none;
}

#list-lines li img {
    width: 100%;
    height: auto;
}

#list-lines::after {
    content: "";
    display: block;
    clear: both;
    overflow: hidden;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>

<ul id="list-lines">
    <li>
        <img src="http://img2.vetton.ru//upl/1000/346/138/vetton_ru_sddu7-2560x1600.jpg" alt="" />
        <br /> Line 1
        <br /> Line 2
    </li>
    <li>
        <img src="http://img2.vetton.ru//upl/1000/346/138/vetton_ru_mixwall66-2560x1600.jpg" alt="" />
        <br /> Line 1
        <br /> Line 2
        <br /> Line 3
        <br /> Line 4
    </li>
    <li>
        <img src="http://img2.vetton.ru//upl/1000/346/138/vetton_ru_sddu7-2560x1600.jpg" alt="" />
        <br /> Line 1
    </li>
    <li>
        <img src="http://img2.vetton.ru//upl/1000/346/138/vetton_ru_mixwall66-2560x1600.jpg" alt="" />
        <br /> Line 1
        <br /> Line 2
    </li>
</ul>