This thread is just asking for flames, but IMO, based on price:performance, problems I've run into supporting systems, materials quality and styling:
Dell (price:performance owns all; quality has improved greatly recently)
Apple (great systems, Powerbooks are too expensive though)
IBM (used to be built like tanks, not impressed with quality lately, have been doing more work on IBM systems than any other. Ugly as sin.)
Sony (don't like to support them because of problems getting drivers, but I love having one as a personal machine)
Gateway (don't see too many of these anymore)
Toshiba (gone seriously downhill in the past 3 years)
Fujitsu (so flimsy)
HP/Compaq (ugh)