You're quite right - a very broad question, lol!
You might have more luck if you specify your question more by what categories of need you have for your store, i.e.:
"What are the best apps for:
Yes, It's actually a broad subject. But the answer depends on your niche, audiences, and some other factors.
Personally, These are the must-have apps for my stores - Trytada, OptinMonster, TrustPulse, Plugin SEO. My favorite one is Trytada. https://apps.shopify.com/tada-pop-ups-game-emails It's almost an updated version of woohoo. It's a gamified email popup app that asks the shopper to play a game to ‘win’ bigger discounts. Tada’s engaging pop-ups will allow you to collect more email addresses.
Give Upsell Wizard a try. Its got a Free Forever plan (which supports 200 add to carts per month) and can definitely help you increase average order value.
You basically just setup which products you want to trigger funnel (series or list) of offers. When a customer adds a trigger product or collection to their cart, they are presented with the offers you setup. This can mean that, instead of adding a single product to their cart, a customer may add 2 or 3, possibly doubling or tripling the amount they spend with you.
It also support conversion tracking and split testing different offers so you can judge which offers generate you the most revenue.
If your store gets a lot of traffic, there are also paid plans that are much cheaper than the competition. Even on the free plan, though, there is no branding from the app so its completely transparent and will let you look professional.