Right side. Sometime I feel like I ought to have it on the left for reasons mentioned above (buffer from traffic?, reflective accents on the pannier...) but I keep it on the right anyways. My reasons for keeping it on the right are that I mount my bike from the left side, so I don't kick my pannier when I swing my leg over. Also, from experience, I have learned that when the bike goes down in a crash, the pannier on the right side is more likely to protect the drive train from damage. Lastly, I have a kickstand, so loading/unloading the pannier on the right side of the bike is easier and keeps the bike more balanced when parked on the kickstand.