I used to love zencart, but unfortunately I've seen it get exploited far too often. Magento is definitely my cart of choice, due to the awesome design of the software and the architecture as well. You do need more powerful hosting, but I've also seen it run well on various shared hosts.
It seems to me that eCommerce has essential lacks. For example, gives out constantly errors of loading of files and it is not always correct uploading the goods