How to return a specific entry?

An alternate way would be to use Twig's |filter() to do the filtering rather than Craft itself.

I can't say how much of a performance difference it makes, but I know that if you're doing very large queries many times, without adequate caching, it could slow your site down if you're constantly making multiple entry queries.

It also allows a bit more flexibility if you're trying to do something non-standard or need a function callback. For example, I wanted to create an array of IDs in Twig and then choose the first entry that matched it, and |filter() worked well in that situation.

{% set featured = [18, 56, 105] %}
{% set entries = craft.entries.section('myChosenSection').all() %}
{% set chosenEntry = entries|filter((entry) => entry.id in featured)|first %}

I realize this question was geared towards getting the entry via twig. Here is how you could accomplish the same from a plugin. In my case, I wanted to find an entry via slug.

MyPluginController.php

public function actionMyAction() {
    $criteria = craft()->elements->getCriteria(ElementType::Entry);
    $criteria->section = 'mySectionHandle';
    $criteria->slug = $this->_getLastSegment(craft()->request->getUrlReferrer());
    ...

    $entries = $criteria->find();

    if ($entries) {
        foreach ($criteria as $entry) {
            error_log($entry->title);
        }
    } else {
        // No entry found that matches criteria.
    }
}

private function _getLastSegment($url)
{
    $path = parse_url($url, PHP_URL_PATH);
    $trimmed = trim($path, '/');
    $tokens = explode('/', $trimmed);

    return end($tokens);
}