I realize I am a bit partial here, but I always prefer to build my own
custom shopping cart system. Granted, I am a programmer, and as
such, I usually whip up a custom system in no time based upon my
exact needs, but I have yet to find ANY shopping cart/ecommerce
system that ever does EVERYTHING I need.
Anyway, as trustdnb stated, it would be nice to know which, if any,
CMS you are using as the answer would obviously depend largely on
that fact alone. Other useful information would be:
What type of features and/or functions are you looking for exactly?
Are you selling ... Physical Products? Digital Products? Services?
Are you running on a *nix based system or Windows environment?
Are you looking for cross-implementation with other scripts/apps?
Also, I noticed from your profile, you are a web designer. Are you
looking for something to use for your clients so that you would
also require some sort of "developer" rights or are you looking
primarily for your own needs, in which case, I'll refer you back
to the questions poised above.