I use the Source Tactical ILSP 2L-3L Low Profile Hydration Bladder. It is almost a square shape rather than the long rectangle of most, it fits in the top lid of my pack and not subject squeezing like when a bladder is in the main pack and also easy to fill. I like the Source Tactical bladders for their toughness, the wide mouth top opening, the built in adaptor that works with the Sawyer adaptors, the Rapid refill adaptor that allows for filling from a water bottle/hose/faucet, and the ridge in the center that keeps them from bugling out and more flat.