I either use an Osprey Flapjack rucksack, or if I am trying to avoid a change of clothes I use a Carradice Bike Bureau. It has some great points and some not so great ones, but work paid for it so I tried it out.
I’m more than happy to ride with a rucksack, I actually prefer it, but because I have to ride to the station then sit for 2 hours on a train, I found it convenient to ride without – that way I’m not too sweaty for the train journey. Also, sometimes the hotel is only 1-2 miles from work so a rack mounted bag allows me to wear work clothes and not have to change. If I am riding any distance, I’ll need to change clothes anyway so I’ll choose a rucksack.
Messenger bag is another option – keep it low down off your back..