You don't mention how much water you want to carry and how much gear (both volume and weight). Those two things will narrow down your choices pretty quickly.
I'm on my second Camelbak Lobo. Plenty of space for tools, spare tubes, some food. The bladder holds 70 oz. It has a flap and bungie cord so I can keep a jacket and tights secure if I need to (although it gets pretty bulky at that point). I also recently got a Dakine hydration pack (don't remember the model) which holds more water (100 oz) and stuff. I picked that up for hiking in the Grand Canyon this past summer. It's bigger than I'm likely to use on the bike. Finally, I cut a slit in my regular daypack and can put a bladder in there now. Comes in handy for picnics in the woods.
Beyond the size, a couple things to look for include the type of valve (I prefer CamelBak's to the Dakine but they both work; the pack's straps - shoulder, sternum, waist; how easy can you take the bladder in and out & can you refill the bladder without removing it from the pack; a large opening in the bladder makes cleaning it a whole lot easier; can you replace the tube and/or mouthpiece.