I have to say BigCommerce. Easy to use, intuitive, has seo built-in, integration with just about every tool out there. Really this thing is a class above all else. It does cost money, but it's money well spent.
Check out my review for more details if you're interested in BigCommerce.
Magento is a great shopping cart to use. It offers complete flexibility and control over the users experience, content, and functionality of their online store. Its intuitive administration interface features powerful marketing tools, a catalog-management engine and is SEO optimized to give merchants the power to create sites that provide an unrivaled and rich online shopping experience for their customers, tailored to their unique business needs.