Here, I will show you the code to get the most viewed products in Magento. The function addViewsCount() filters the products with their views count.

Here is the code:-

Get overall Most viewed products

public function getMostViewedProducts()
{
	/**
	 * Number of products to display
	 * You may change it to your desired value
	 */
	$productCount = 5; 

	/**
	 * Get Store ID
	 */
	$storeId    = Mage::app()->getStore()->getId();       

	/**
	 * Get most viewed product collection
	 */
	$products = Mage::getResourceModel('reports/product_collection')
		->addAttributeToSelect('*')
		->setStoreId($storeId)
		->addStoreFilter($storeId)
		->addViewsCount()
		->setPageSize($productCount); 

	Mage::getSingleton('catalog/product_status')
			->addVisibleFilterToCollection($products);
	Mage::getSingleton('catalog/product_visibility')
			->addVisibleInCatalogFilterToCollection($products);

	return $products;
}

Get Most viewed products for current category

public function getMostViewedProducts()
{
	// number of products to display
	$productCount = 5; 

	// store ID
	$storeId    = Mage::app()->getStore()->getId();       

	// get most viewed products for current category
	$products = Mage::getResourceModel('reports/product_collection')
		->addAttributeToSelect('*')
		->setStoreId($storeId)
		->addStoreFilter($storeId)
		->addViewsCount()
		->addCategoryFilter(Mage::registry('current_category'))
		->setPageSize($productCount); 

	Mage::getSingleton('catalog/product_status')
			->addVisibleFilterToCollection($products);
	Mage::getSingleton('catalog/product_visibility')
			->addVisibleInCatalogFilterToCollection($products);

	return $products;
}

Get Most viewed products for last 30 days

public function getMostViewedProducts()
{
	// number of products to display
	$productCount = 5; 

	// store ID
	$storeId    = Mage::app()->getStore()->getId(); 

	// get today and last 30 days time
	$today = time();
	$last = $today - (60*60*24*30);

	$from = date("Y-m-d", $last);
	$to = date("Y-m-d", $today);

	// get most viewed products for last 30 days
	$products = Mage::getResourceModel('reports/product_collection')
		->addAttributeToSelect('*')
		->setStoreId($storeId)
		->addStoreFilter($storeId)
		->addViewsCount()
		->addViewsCount($from, $to)
		->setPageSize($productCount); 

	Mage::getSingleton('catalog/product_status')
			->addVisibleFilterToCollection($products);
	Mage::getSingleton('catalog/product_visibility')
			->addVisibleInCatalogFilterToCollection($products);

	return $products;
}

Hope this helps and thanks for reading.