I love my JuJuBe-Tween. It is deceptively roomy without seeming too big. I can easily fit 4 nappies, wipes, wetbag, change of clothes, purse, drink and snacks. I bought it pe-loved from here which is an added bonus!
I also have a planetwise sports bag which is handy, it has a built in wet bag, and I can fit loads in there too, and when Dot is out of nappies she can use it as a swimming bag.