Non-drive side.

The side I mount and dismount on.

Road side for perception of drivers having better visibility. I put an extra flasher on the pannier strap in the dark/wet months.

If I have one on the front it will be non-drive side as well. That is only based on balance.

I am left handed, I noticed [MENTION=336334]the sci guy[/MENTION] mentioned that. That may only play a role for my front pannier through.

I was presented with mounting on the drive side the other day and I did not like it at all; felt weird, cumbersome, awkward and I ended up just switching sides anyway to mount.