CamelBak M.U.L.E. NV Hydration Pack - 854cu in.
this pack rocks. plenty of room for all day rides but a great suspension design keeps it light on your back. i've been riding all kinds of stuff from XC-ish longer rides to lift served laps without feeling the need to take it off. the back panel keeps you well ventilated, the shoulder straps allow a great range of movement, and the helmet carry while designed for regular lids doubles as an awesome fullface carrier. get one!