I would have to agree that Magento is the most powerful just purley because it is designed to do anything you throw at it. The only problem with Magento is that it has a very complicated file structure so be ready to have to pay a coder if you need even the smallest thing changed or tailored to your site. I have been studying magento design and coding for over a year now and I'm still only regarded as a complete novice!
I have used Magento for mine and a couple of people that I know use it as well. I can't tell you about others because I really haven't tried them. But Magento has never given me a reason to try anything else. It is very easy to understand and handle.
For Magento users...can it: Have some type of 1 page/speedy checkout? Or automatic account creation upon checkout? Guest checkout for people who don't want to make accounts? Checkout with email only for downloads when using something like Paypal? (people usually have to fill out their info there anyway, and make them fill out something twice and they may get bored and leave) Allow download and/or shipping options for a single product? (most stores aren't clear on this and you must try them first) Or do I have to have 2 entries for every title that I have a download and DVD of? These are the challenges I've yet to overcome in trying to find a shopping cart for a store that deals heavily in both downloads and shippable DVDs.