What's your budget, a regular roomy bag, with a large handle to drape over pushchairs, with a travel change mat works just as well. A friend uses her massive shopper style handbag as a changing bag and it works just fine for her.
I don't really have a budget, but wouldn't want to spend a large amount. I have a old fashioned Silver Cross too so would have to have adjustable straps to fit over the big handle or be a handbag style so I can use bag clips.
I'm planning on just using a skip-hop backpack with the changing mat from the free boots bag. I reckon it will last a lot longer then an actual change bag which I find crazily expensive for what you get and how long it is actually used for x
Bought an ordinary rucksack from Millets which had lots of room and decent straps so was comfy to carry on my back. Used for 2 babies then for normal family outings/shopping/etc. Only retired it last year when oldest child was 11
I love my babymel bags, very stylish and roomy. Mothercare tend to sell them half price in the jan sales but I've also bought one from eBay. I put it in the wash and it came out like new. Stay away from yummy mummy bags, I found my first one to be very heavy even without stuff in it!
I have a yummy mummy rucksack (it does not have any naff logos/pictures on it thankfully), that I got off eBay. It has three zipped sections, two outer pockets and loads of inner pockets. It's perfect for me as the baby is in reusable nappies which take up a fair amount of room. There is room for a change of clothes for the 3 year old and the baby and all the other gubbins I need for a day out (we had a picnic for 4 in there too in half term!). It doesn't weigh a ton either. Have a look on eBay!