I'm sure you will get as many different choices as you get answers. I recently purchased this bag:

http://63.110.6.56/index.php4?page=s...category_id=16

(the link is to the manfg's website) and really like it. without expanding it can function as a carry on, and for three or four day trips it's been able to carry everything I need. It's proably the best organized bag I've had to date.

They list it for $305, it sells for considerably less then that at places like Magellans and other online retailers.