Shopping Cart Service, Which is best?

I have an OsCommerce/Cre based cart at the moment that I have spend a lot of money improving but still seems useless after all with it's checkout abilities and customer service management.

Looking for one that has amazing capabilities at ease:
1. 1 page checkout.
2. Customer can obtain RMA by himself.
3. Customer can download a shipping label for his RMA.
4. Has many of the oscommerce contribution with less mess.
5. Navigation user friendly.
6. SEO friendly.
7. Handle 1000's of items at ease.
8. Handle many item Contributions with variable weight and etc'
9. Have integration to or programs supporting financial software such as QB.
11. Supporting multiple sites inventory.
10. And etc', and etc'

Any recommendation and explanation why would be appreciated.

 

Check out Magento magentocommerce.com

 

Yes, I know magneto but looking for one that is much more powerful. Doesn't have to be a free one!

 

hi.

i am magento freelance developer

 

And you are trying to imply what?

 

take a look at interspire shopping cart - their software should offer what you are seeking.

 

We used CS-Cart. It has most if not all the things you are looking for. I really liked CS-Cart.

 

Zen Cart and Magento are rich-feature php shopping cart, they easily integrated in any running or new ecommerce website.

 

I am using Zen Cart