Magento Price Rules – Which Price or Rule Has Highest Priority?

The code that drives the price can be found in Mage_Catalog_Model_Product_Type_Price

public function getFinalPrice($qty=null, $product)
{
    if (is_null($qty) && !is_null($product->getCalculatedFinalPrice())) {
        return $product->getCalculatedFinalPrice();
    }

    $finalPrice = $product->getPrice();
    $finalPrice = $this->_applyTierPrice($product, $qty, $finalPrice);
    $finalPrice = $this->_applySpecialPrice($product, $finalPrice);
    $product->setFinalPrice($finalPrice);

    Mage::dispatchEvent('catalog_product_get_final_price', array('product'=>$product, 'qty' => $qty));

    $finalPrice = $product->getData('final_price');
    $finalPrice = $this->_applyOptionsPrice($product, $qty, $finalPrice);

    return max(0, $finalPrice);
}

In _applyTierPrice

$finalPrice = min($finalPrice, $tierPrice);

and through _applySpecialPrice

$finalPrice     = min($finalPrice, $specialPrice);

There are no options involved and it will return the minimum price.

In my view this also matches the expectation of the customer!

If you do want to change this behaviour you can observe the event catalog_product_get_final_price and use $product->setData('final_price',X.YZ); to the value you want.

There is no way to determine for sure what is the order of the rules if they have the same priority. In this case the rules are retrieved as MySQL want's it and it may not be always the same order. The order can be different depending even on the MySQL version. See this answer here (and comments). These guys really know what they are talking about. I had the same issue/question before and during tests I got strange results for different items in the cart. I've even got lost in the numbers when I had combinations of rules that were applied on a specific item and on the full cart.
Conclusion Always specify priorities to rules that might affect the same product.