My recommendation would be ZenCart, I prefer it over osCommerce.
Reasons:
- More active development, bug/security fixes updated frequently, new major releases in every few month.
- The only valid XTHML open source shopping cart so far. (I didn't know any other)
- Tableless, layout controled by CSS.
- Coding and template are separate (good for designer)
- Easier for future upgrade without breaking every code of your existing customization.
- Override template system, you never touch the original code, instead you work on separate custom directory, so if something goes wrong, just swicth back to original by clicking a button.
- More built-in features