One that fits you. :D
Really, though, I'd suggest finding a good outdoors store that has a wide selection to check out some packs first hand. Try them on, see them up close, pricing, etc.
I shopped Eastern Mountain Sports, REI, and Hudson Trail Outfitters, checked out the bags, picked the brains a little there, then bought mine. My pack retails for $200, and the best online I could find was around $150. Searched Ebay and nailed it brand new for less than $120 shipped, IIRC.
BTW, mine's an Eagle Creek Transcontinental Journey.
For that matter, sift thru some of the older threads here, theres a ton of info on which pack, etc etc there. I would say that the TCJ that I have is a bit big for someone your size (I'm 6ft/185lbs)...