I really like my Lowepro Flipside 400 AW. Holds a gripped body and several lenses comfortably - last long trip, a gripped 5DII, 16-35L II, 24-105L, 70-200L II, TS-E 24L II, 35L, and 430EX II). The pack is comfortable enough to wear all day with a load, and has an all-weather cover. The main compartment opens in front (the part against your back) for extra security from casual opening/theft, and the design allows you to remove the shoulder straps and spin the pack to the front on the hip belt, where it serves as a lens-changing platform, and you never have to set the bag down on the street or in the mud.
For air travel, the loaded Flipside 400 AW fits snugly in a Pelican Storm im2500 hard case - allows rolling through the airport and more importantly, hard protection if you're forced to check your gear.