CI is best for shopping cart ?

You can use every Framework you want for a shopping cart libary, even pure PHP. So it depends on your personal taste. Of course CI is a god choise for such a project. You know that CI provide also a simple shopping cart class http://www.codeigniter.com/userguide3/li.../cart.html ?

Another nice cart library you will find at http://haseydesign.com/flexi-cart/

And a more complex ecommerce system based on CI you will find at http://www.heroframework.com

So it's up to you and your welcome to CI ;o)