I'd recommend Ergon – I really used to suffer having to lug varying amounts of kits too and from work every day – tried a variety of rucksacks / courier bags etc but nothing has come close. Really adjustable, weight sits nicely down on the hips, the ball joint keeps things very stable and allows plenty of freedom of movement – there's also a wealth of small details like decent routing for internal bladder, lots of little pockets, all pads are removable for washing and it has a rigid frame which means you can lash things to it…
My BD1 has seen about 7000 miles of commuting and is still going strong.