Just pick one that you like. I bought a Coach diaper bag from the Outlets that doubles as my purse when we go out for long periods of time, and will be a good work bag once I no longer want a dedicated baby bag. But probably 50% of the time we go out, I don't bring anything, just throw one of those wallet style changing pad things that hold 2 diapers and a small box of wipes in the basket of the stroller, sippy cup in the cupholder, and clip a stuffed animal to her stroller straps.
I've never been one of those "I need to carry every possible item with me!!" type of frantic moms. When she was a newborn I packed an extra bib & burp cloth with me (she had reflux & was a spitter), my nursing cover, and a backup pacifier. When I transitioned to formula, I thew in a couple empty bottles (or 1 premade), a little Munchkin divided formula cup thingy with the powder, and I'd buy a bottle of cold water to mix it on the go.