A lot of this depends on what exactly your client needs, of course. But I thoroughly researched this recently for one of my clients and for my family store, and would put CoreCommerce at the top of that list. User-friendly, easy to customize, and tons of features for the price. Owned by Sum Effect, the folks who sell DigiShop (as the licensed version). I used DigiShop for a former store and liked it.
I put the family store on BigCommerce, which is the hosted version of Interspire. It runs pretty much neck and neck with CoreCommerce, in my opinion. It's new, though (the hosted version, not Interspire). Customer support has been good. Very easy to use, and the template coding is the cleanest I've seen on any cart.
To be honest, I thought ShopSite was too expensive for the features it offers. Ditto for Volusion, Pinnacle Cart, and some of the others.
Hope this helps.