Well,all of them are good.But I advise you to use OS commerce.It seems that it's more popular.And then you will have more solutions to deal with it when trapped in some problems.
They all have they advantages and disssatvantages so it depends on your needs. If you are looking for and easy to use plug and play solution I can recommend the Retailer eCommerce Shopping Cart. Click my signature link to check it out.
they are all fantastic for a small shop that is more concerned about bootstrapping startup costs. I really don't care for prestashop, its a nasty mess (*ask me about writing dynamic shipping modules) for GPL systems (*no not free, nothing is free - think of it as deferred cost instead) I highly recommend magento Why? It has the mature features an e-commerce site should.
i think both zencart and os commerce are good. but what are you planing to do with these two scripts? if it`s not a secret of course.
you can choose one from zen cart and oscommerce in terms of user friendlyness as well as simplicity to modify the code.
Have you read the blog entry athttp://www.oscommerceuniversity.com/blog/?p=201 about magento import failures?
Our team use OS Commerce and we would say it is great. I'm ignorant about the other and so - NO COMMENTS on them!