Only use plugins that you absolutely need for business reasons. Your choice largely depends on what (and how) you are selling, but I tend to go for mods that make the customer experience faster and easier...
So, anything that loads the site faster, such as image handler, and numinix's FEC (with CJLoader to streamline jscript and css).
I have also got responsive templates working on my major sites, as over 50% of my visitors are shopping on hand-held devices now.
For admin and product management I use EasyPopulate and on some sites use Apsona ShopAdmin.
I have also worked on the product info PHP files to accommodate schema.org microdata and "google rich snippets" functionality, and the principles of purl.org/goodrelations (these functions are now paramount if you want good SEO.)
Most of the other plugins I avoid - unless they prove to be necessary for business reasons or an enhanced user experience.