i'll do my rateings from personall experiance in both repairing, and calling them for warranty repairs for customers.
worst off is dell. their machines under $1000 are crap. they make up (by they, i mean those under $1000) make up about 75% of our sevice department, altho i'm sure this is partly because most people are under the impression that they are the best out there.
second worst is emachines. they would be worse than dell, but their tech support is better.
next is gateway. in my house, and at work, i've seen them use crappy parts, and reformat hard drives without telling customer's what they are doing, only to piss off the customer when their data is gone. Dell has done this too, but not as often.
next, alien ware. Owned by dell, but higher end. typically, i don't see many of these, but when i do the problems are catastrophic. tech support are teh same people, jsut different note cards taht they are reading off of.
now things start to get better
i will start with compaq. good systems but tech support sucks.
HP, better than compaq. Tech support is getting better here. worst company i've delt with that has acutally had US tech support (sometimes.)
here i will put sony, toshiba and any other (besides the 2 after). I put these here because i havn't had enough experiance to give a good review. The occasional tech support calls go better than most, and the hardware is decent quality.
Best mainstream in my oppinion would be Acer. Of the countless times i've called them, i've always gotten an American on the other end. Unlike just about every other company, their support personal do not read off of a script, and don't keep transfering you to techs higher than themselves. Odds of you calling them tho, slim. Their equipment is top-notch and good quality.
Finally would be the small-chain/self-built computers. Self-built is obvious. You can call yourself and get tech support, and you know the quality of machine you are getting before you build it. Small chain stores generally build quality machines. they need too to keep you from going to the big-gun manufacturers. You will also see that most have in-store techsupport so not only do you have the option to call them, but you can also talk to them inperson.
like i've said, this is all from personal experiance working with these companies at work. none of these are just "i've seen one bad dell, so they are all bad". its from an overall perspective of what i have seen in their products.