You will always find that it is cheaper to build your own desktop PC rather than buy from one of the brands I just mentioned. Some companies like palicomp and CCL sometimes have deals for pre-built desktop PC's which are about the same price as building it yourself.
Regarding both desktops and laptops, most big businesses will get from one of those three brands I mentioned. Lenovo (Thinkpad T, P and X series + thinkcentre), Dell (OptiPlex 5 and 7 series and Precision 5 and 7 series) and HP (Probook and Elitebook).
On the consumer range, the top gaming desktop and laptop models for each brand are Alienware (Dell), Predator (Acer), Legion (Lenovo), Omen (HP) and ROG (Asus) but these are expensive. Obviously Apple has their own products which are all pretty good but expensive as well.
Sometimes models will have features which are universally panned and other times praised. Sometimes the manufacturers will change something significant and make certain models they sell better or worse. The only way to know for sure is to read unbiased reviews for each product.