I like ShipStation. Really wonderful for shipping.
Product Customizer is really helpful for adding lots of variants. I ran out of the ones allowed by Shopify pretty quickly. There is a new one out by the people that do Product Upsell (Shappify) that seems to do the same thing and is about 1/2 the price. Looks good but I haven't tried it yet.
Lexity is a free app. that shows activity in your store. Good stats on when someone is in there, what pages they looked at, time spent, etc.
You can never go wrong with making your store mobile friendly. Just installed a mobile theme for my store. I gotta say I am pretty proud of it. Looks even better than the main theme.
Dropifi is also another great app. It allows your customers to drop a question or a compliment with 0 hassles.