Well, this is a question we can't answer as people have different needs, and what's out there is continually changing.  However, some people spend all their time testing and reviewing computers - we recommend you check out some of their sites for computer buying advice:

Desktops

• http://review.zdnet.com/4566-3118_16-0.html
• http://www.pcmag.com/category2/0,2806,4,00.asp
• http://products.howstuffworks.com/desktop-computers-types.htm

Laptops

• http://review.zdnet.com/4566-3121_16-0.html
• http://www.pcmag.com/category2/0,2806,9,00.asp
• http://products.howstuffworks.com/laptops-types.htm