My favorite is actually a back-pack "valet" diaper bag - it has one large compartment and a second section that zips all the way open so the bag lies flat. It's designed to attach onto the back of the front seat of a car and still have all the little pockets handy. I originally bought it years ago when DD #3 was little enough for diapers. It's the best because:
1) waterproof - tested through water rides and soaking Florida rainstorms (although I do keep a large ziplock baggie in there when I'm in the parks for cell phones, cameras, etc on water rides as double protection in case the zippers ever leak. Hasn't happened yet though). Wet articles in the bag leaking through to the outside aren't an issue and if anything spills it's so easy to dry it off.
2) easy to clean - it's dark blue so it never looks completely ratty from being kicked under seats but a quick wipe down with a damp paper towel is all it needs for "freshening up".
3) lots of pockets for little things - and they are see-through so they don't hang you up through security.
The large main compartment is big enough to hold four foldable rain jackets, dry underwear/shorts in case needed after Kali River Rapids, ziplock baggies to protect valuables against wetness, etc. while the pockets for baby bottles now hold a couple of water bottles, the ones for diapers now holds the "important disney information" notebook, etc. The kids bring smaller back-packs for their pin lanyards, autograph books, pens, etc.
6 years on and I couldn't imagine taking another bag. Mine was made by Fisher Price but I don't know if they make it anymore.
