Definately ride a Specialized. They have a sweet "complete" package. Good mix of components and a great quality frame.
What about the Giant? Their new line of bikes continues to impress me. Since they weld their own frames, (an a lot of other companies) they are usually a bit cheaper or come with a better component spec.
Gary Fisher's and Trek's are good as well.
Diamondback isn't what it was 5 years ago. They seem to have dropped their higher end stuff, but a good solid bike.
Don't forget to check out Jamis, KHS, and Ironhorse. All putting out some sweet bikes these days.
If you want a good solid bike for the money, but can deal with a little more heft, check out Haro Bikes. Good bikes, very durable, cheap, but a bit heavy!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger