When I had Rilo, I bought a Petunia Pickle Bottom Boxy Backpack as my diaper bag. I really liked it...it had a lot of space, a fold out changing pad and backpack straps, which I found super convenient when I was carrying Rilo + a million other things. After carrying that around for a year and a half, I went back to a regular purse.
Now with Collier I'm in need of a new diaper bag again. Really, any large purse will do, but I hunted down some of my favorite actual diaper bags. I've fallen in love with the line by Timi and Leslie (no.2 and 3). I love the accessories and designs of all their bags.
Here's what I look for in a bag. 1. Lots of room...snacks, books, blankets, bottles, etc. plus all your normal purse items are all going in this bag. 2. Pockets, and preferably some zippered pockets to store things I don't want to dig around for. 3. Diaper changing pad (you can purchase these separately but I find it convenient to have one with the bag). 4. Stroller straps (or some way to hang the bag on my stroller)
