If you could start any app that would automatically be successful, what would it be and why?

Sell this opportunity to someone else

If I knew such app, why would I tell you or anyone about it ?

But to answer your question, how about a facebook app ? Why ? It has already been proven to be successful 😎

A borrowing/swapping app. Board games, jigsaws, video games, books etc. Set up a local group, invite people (can be rated on trust and how well they return stuff), have an inventory of what is available and who currently has it, pick-up and drop-off spots.

Could also work for power tools, but might need a deposit system to limit damage/theft issues.

Data Management App and removal App.

Specially concerning the privacy of people.

Or

A central knowleadge base where all human made is stored and organised for easy retrieval. Saves a lot of time and improves innovation as redundant work is minimised.

An app where you put your bank account number and money starts flowing in. It would be expensive to maintain, though. But definitely popular.

auto app developing app

@mAndroidDev yooo. You beat the system

Also like the idea of a group-based betting app. It essentially holds money until the result is presented but it's can be any sort of bet - no matter how ridiculous. When a group of friends decide to make a bet they enter the terms, who can join, how much can be bet and when the bet is taking place. It locks the money in, plus an additional deposit from each member. All must confirm the results of the bet in order to release the money and to pay the winner. One team member acts as resolver, in the event of a disagreement they will make the final decision.

AR measuring app

you hold the camera over the thing you want to measure, maybe select points or corners and boom here's your number.
maybe with basic math like: that width minus the center of that board...

Even better if let's say you want to install a kitchen top and already got the other stuff in place. so you AR that shit and tell it I want this board in this corners on that drawers - how do I cut the board?

Bonus if you can take notes in the form of pictures and generated CAD sketches and for example align an old picture of sth with how it now looks and it'll automaticly copy the Datapoints from old to new