WooCommerce & Jigoshop are built on the same foundation so they approach things the same way, but WooCommerce is likely the winner because of the amount of sub-plugins and resources. I just finished a WooCommerce project and really like how it’s coded as well as its usability.
I haven’t used WP E-commerce in a long, long time … but I don’t ever plan on going back.
Shopp has had a buggy history, but with their latest update I’m about to upgrade a client and looking forward to the tighter WordPress integration. Has plenty of extensions as well.