Answering the question is trickier that it sounds.
Shopify marketplace provides apps (or plugins) under several categories ie marketing, sales, social media, shipping, inventory. Under each category, stand some must have Shopify apps most of which are free to try for 14 or 30 days.
A quick biased answer would be those 5 apps:
But I would say that the most useful apps are the apps that help you scale your business.
Growing your business is always good.
You need to look at the bottom like number not just how many visitors your store have but how many sales you have.
But if you get 1000 visitors to your store per day at first and then increase to 10 000 but don't work on improving your conversion it will be a very expensive exercise.
What I suggest and what any store owner that have a successful business will agree on is to make sure that you get as many of those visitors to become paying customers and buy from you at first.
The first step before generating more traffic should always be to make sure you have a high conversion rate.