If you want to carry a helmet, bike-lock, and hydration bladder, the Deuter Attack Tour 28 cycling pack ($225) is a solid performer.
This pack also protect backs (large sized 44-54cm or small sized 38-48cm) during mountain-biking, city-commuting, and touring in the event of a fall. I use it while longboarding. Its thick padding, which is stored near the back-plate, provides excellent peace of mind and can be thought of as a very inexpensive insurance policy. Thankfully, the outer plate that rests against your back is extremely ventilated with lots of mesh throughout so the pack doesn't become soggy when worn while riding on a muggy evening. There are 10 ventilated components: 2 in the shoulder straps, 2 in the hip-belt, and 6 in the padding along your spine.
The Attack 28 is one of 9 H20-Ready packs that Deuter offers including the Attack Enduro 16, Attack 18 SL, Attack 20, Race EXP Air, Compact Air EXP 1-, Compact Lite 3L, Compact Lite 2L, and the Race X. Its hydration sleeve compartment is housed closest to the back plate with a velcro hook for hanging your bladder. The official reservoir, the Streamer 2.0 ($32) and the Streamer 3.0 ($34) fits into the pack precisely.
The storage options provided are immense. The bottom of the pack holds a mesh net which can be stowed but can also spring upwards to secure your bike helmet to the top of the pack with hooks, and mesh water bottle pockets on both sides of the pack, alongside two zippered pockets on the sides of the hip-belt. Next, is a compartment behind the Attack 28 Tour logo which has a mesh zippered sleeve. On the other side are two mesh elastic secured compartments with an additional two non-mesh elastic sleeves behind and a key ring holder on a cord. Above is a sunglass-sized pocket with a soft microfiber tongue. Behind the sunglasses compartment is the largest pocket which holds a zippered pocket, also about sunglass-sized. Behind is a generously sized, wide elastic sleeve which is about the width of three hands stacked upon each other, with a large rectangular black mesh window above. The final pocket features a piece of velcro at the top to secure your hydration bladder in place and a neoprene opening to feed your tube and bite-valve out of the pack. The fabric has Alpine warning signals to aircraft: "Yes We Need Help" and a logo of a man making a Y with his arms.
Deuter did not spare expenses with its materials. The 600-thread polyester outer shell, which requires a serious 28-30 needle, measures 1mm in diameter. Fun fact: this heavy duty thread—the heaviest thread generally available for purchase by consumers for personal projects—is used for hand sewing, machine sewing, and for tying, binding, and hanging. In addition, the shiny metal clasp at the waist is also a welcome addition and feels more sturdy than the plastic clip buckles found on most packs. The visco-elastic SC1 perforated foam with circle cut-outs also known as the SAS-TEC Body Protection System, makes up the Deuter Shield System.
If you have a lot of cycling gear to tote around, the Attack Tour 28 is worth a shot.