First you want to decide if you want open-source vs a closed platform. As a general rule open source need more work and at least some technical knowledge, but there are cheap/free and you have complete freedom with them. Closed platforms are generally easier to setup and easier to maintain but are more expensive (up front costs at least) and because its closed you have little freedom ( at the mercy of the company the develops it)
Note that is a very general rule and every platform is different and there is always the argument (normally put forward from closed platforms) that open source is can be expensive in the long term because the costs of upgrading/maintaining it, espically if you our sourcing that work out.
I have used Zen-cart alot. big bonus is its free, its got a nice community (but not huge) and its a decent platform, but can require a bit of work to get it the way you want. big negative is that it can't do stock control for products with options eg you have a shirt that comes in an option of red or black, it just tracks the shirt stock not the amounts of red and black shirts. If you are not using it for stock control then its not an issue (as I believe they are working on a fix for this) . Over all Zen-cart is a good start if you have a low budget.
Starting to use Magneto, and even though its also "open source" it frees alot more commercial than zen-cart, but has a far bigger community and tons of extensions. It still need a bit of work to get setup but it alot more flexible than zencart and has more 3rd party modules. There is a reason its the biggest E-commerce system in the world.
Never used Open-cart, but I did look into it and it looks nice ( but a know of a competitor that moved from open-cart to magento enterprise)
Another one I looked at is visualsoft, people that I know use it are happy with it as is easy to use and because it a closed platform you don't have to worry about alot of the technical stuff. The basic price for it is ok, but they really get you with the addon and can soon add up on price (that and you don't have the freedom of opensource is the reason I did not go with them)
I'm sure there are more platforms that are just as good if not better that the ones above, but I can only tell you about the ones I have experience with 