Depends on the purpose, and the level of the user.
For gamers, I HIGHLY suggest Alienware or VPRMatrix (Best Buy brand). They both use the best components
IMHO, and their support is only matched by Dell.
Dell is a great company for the novice to mid-range user or mutimedia type person. Excellent support, and great built systems. Gateway beats Compaq and HP
IMHO, but they are slimming down operations.
I was turned down from IBM PC's a while back when I bought an Aptiva for my mom, and later found out the only memory I could use for upgrading, was proprietary to IBM, and had to be bought from them. They have probably stopped this horrible practive (Compaq did it too), but I won't be buying one anytime soon.
For desktop graphics and/or multimedia heavy users I recommend buying a Sony built PC, as they cater to the elite multimediaist (That a word?), and have some great (Although more expensive) pre-built systems.
I of course can always admit that building your own PC can often save you money, and give you the confidence of knowing your parts/machine. However, pre-built systems have come down in price, and gone up in quality. I have recently found it is nearly as cheap, if not just a few dollars more to buy a VPRMatrix brand PC, get name brand insides, and add-on as I need. I actually spent $10 less for a more powerful system then I could have built myself at the time.