I recently bought an Ortleib Bike Shopper single pannier, and I like it. The mounting system is amazing - no amount of bumps will even move it - but then just one tug on the handle and it comes right off. The closure is like a zip-loc bag, rather than a roll top, but it's totally waterproof and air tight. It has enough space for clothes and some groceries for on the way home. And don't worry about balance with the single pannier, once you get rolling you'd never notice. It cost about $100 from REI.

Edit: just saw you're from Canada, so no REI, but I'm sure you can find a dealer/order direct from Ortleib.