I love my Isoki nappy bag.
I had an OiOi, which I also really liked, but the zip gave out, I think cos it was plastic.

The Isoki is a bit heavier than most because it's made from a really sturdy material and has metal zips, but it fits all I need for two kids and has a good little nook underneath for my nappy stuff and other nooks too. And it's reversible, if you ever want to change from a solid colour to pattern or vice versa.