Backpack 95% of the time. The convenience of them outweighs any of the other options for me. We aren't allowed to take bags into work, so, a backpack stows easily into the work supplied lockers. Backpacks, depending on design (I use an Osprey Momentum 30), allow for much better organization of my stuff than any pannier or trunk bag. Zero theft worries because when I get off the bike, my backpack goes with me. Also, panniers add more wind resistance as my backpack is normally in my body's wind shadow. The other 5% of the time, I use my backpack and a milk crate. If someone steals the milk crate, I just 'borrow' another from behind the grocery store.