Example mofule using incognito field - "favourite category". More info: http://craftsnippets.com/articles/using-incognito-field-plugin-for-craft-cms

gistfile1.txt

// place this code in module variable file
public $userFieldHandle = 'visitedarticles';

public $categoryFieldHandle = 'articleCategory';

public function saveArticleVisit($article){   

    $user = Craft::$app->getUser()->getIdentity();
    if(!is_null($user)){

        $visitedJSON = $user->getFieldValue($this->userFieldHandle);
        $visitedIds = (array) json_decode($visitedJSON);

        if(is_null($visitedIds)){
            $visitedIds = array($article->id);
            $user->setFieldValue($this->userFieldHandle, json_encode($visitedIds));
            Craft::$app->elements->saveElement($user);
        }

        if(is_array($visitedIds) && !in_array($article->id, $visitedIds)){
            $visitedIds[] = $article->id;
            $user->setFieldValue($this->userFieldHandle, json_encode($visitedIds));
            Craft::$app->elements->saveElement($user);
        }

    }
}

public function getMostVisited(){

    $user = Craft::$app->getUser()->getIdentity();
    if(!is_null($user)){
        $visitedJSON = $user->getFieldValue($this->userFieldHandle);
        $visitedIds = (array) json_decode($visitedJSON);

        if(is_array($visitedIds)){
            $entries = craft\elements\Entry::find()->id($visitedIds)->all();
            $categoryIds = [];
            foreach ($entries as $key => $entry) {
                if($entry->{$this->categoryFieldHandle}->exists()){
                    $category = $entry->{$this->categoryFieldHandle}->last();
                    $categoryIds[$key] = $category->id;
                }
            }

            if(!empty($categoryIds)){
                $count = array_count_values ($categoryIds);
                arsort($count);
                $keys = array_keys($count);
                $popularCategory = craft\elements\Category::find()->id($keys[0])->one();
                return $popularCategory;                    
            }

        }
    }
}