Threads on Zen Cart forums about recommended modules come around very often, but what is most intriguing about them is the wide range of "must-have" modules and the disagreements about what they are.
Since modules (plugins and addons) inhibit and raise the cost of upgrading, I strongly recommend that you use them only to solve problems or needs that you have on your site, to make your management easier and/or faster; and not because somebody else thinks they're a good idea or solved something for them.