oScommerce will be ideal one, whatever your need is, modules will be already developed and contributed by experts. Zen cart , Creloaded, magento etc are also good. Contact us if you like to get our help.
In fact, there is not the best free shopping cart in this world. The most familar of shipping cart you fee, the best it will be. For me, I will prefer to select the shopping cart that will offer me a lot of information which will include some basic knowledge or some main poits. I am a user of both Oscommerce and Zencart. Both of them are very good. My suggestion is that ask your friend which shopping cart they used. Then try to select those one which was used mostly. Your friend will give you some important suggestion once facing some problem.
If you have the patience then go with Magento, if you are starting off and want a quick solution go with OpenCart.com
yes! oscommerce was free and easy manage I have many template bought from algozone and template monster, let me know if you interested Regards