High end mainstream: Alienware is total crap (way too much flex in case design) Dell's XPS line is decent though not the best.
High end but not very well known: Killer Notebooks will give you everything you want quality and performance (though it will cost you)
Low end: either Dell or HP would be my choices.
Mid range: Asus with their G1
HP is probably the all round best though their video cards max out at the Go 7600. Dell offers a good combination for price to perfomance with the Inspiron line (try to avoid the XPS, they underclock the 7950 to avoid heat issues). Gateway I have little experience with though I've heard both good and bad. Asus has decent quality as does Acer. Try to avoid companies like IBuyPower as I have heard horror stories. Give a price range for the best advice.