We travel quite a bit--and having a good diaper bag is a huge priority for us. We have a few--the Lands End Do It All Deluxe (took it on the plane as very new parents of a 6 week old--a 20 hour journey--and we've never taken it again), a very nice and fairly light Coach microfibre/leather bag for "lunch out and shopping with the girls and the baby"(which is perfect for that reason!)--but the one that goes on the planes, trains and automobiles is the One Step Ahead Backpack. It is PERFECT for travelling. We have learned that having a diaper bag that goes over the shoulder while travelling is quite troublesome--sore shoulders--and when bending down, you could accidently whack your baby with your diaper bag! And it is so helpful to have your hands free with babies, boarding passes, strollers, car seats, snacks, bottles and toys to juggle all at once.
I also find that having a bag that zips completely shut is important when travelling.
So I highly recommend the One Step Ahead bag--the diaper stuff has its own little area so you don't have to open the whole bag--great for airplanes when you have little room for fumbling for things. It can be a bit large if you pack it up which we do (DS' blanket and stuffed travel pals take up quite a bit of room these days)--but it's fine once it's on my back. It fits Avent/sippy bottles/snacks with its huge side pockets (insulated) and has many other little pockets inside.
I never travel without it!