Write what you enjoy.
Even professional authors have written first books they couldn't sell, and even when famous wouldn't sell without rewriting them from page 1. It is difficult to sell a first novel (but not impossible, it has happened more than once).
If you are not writing something you love writing, chances are you won't sell it. Like it or not, your ambivalence or disdain for the story will come across to readers, you won't care enough to put in the hour of creativity it takes for every page to avoid the clichés and tropes and formulaic plotting. Readers can feel mechanistic writing, it lacks emotion because none is felt by the author.
If you don't love your story and are only interested in the money, this is a dumb idea. First novels often get advances of $3000 to $5000, and never earn them out. They typically sell less than 1000 copies in two years. For publishers, if they put out enough new books they make money on the best sellers; it is a matter of betting $3000 on ten books expecting nine losses and one $50,000 winner (plus relationships with authors that promise better results next time).
That is very much worth doing for the publisher, covering their costs and earning them a good salary, but it is actually not that great a bet for authors: 12 months of writing for $3000, total?
Bestsellers have typically struck a nerve in a new way, and it is typically only well-established writers known to already have a following that are going to be able to cash in by striking that same nerve. Not new authors with no following writing something derivative.
For new authors, it is very important to live on the near frontier: Not so original as to be opaque and difficult to follow, but also not so pedestrian that you are competing with established authors that do a far better job than you of pedestrian fantasy, mystery, romance, horror or whatever genre you write.
In other words, you must be original but not too original. Like Sanderson, writing very accessible Fantasy but with a new take on Magic Systems.
Like JK Rowling, writing a book accessible to children but with an adult-level mystery embedded in it.
Like Dan Brown, writing a typical puzzle mystery with a very original Setting (Church and Vatican lore), with Biblical figures as the topic of the mystery.
Like M. Night Shyamalan in The Sixth Sense, taking a pretty much traditional Medium that can see and talk to dead people, but imagining their early childhood, and settling on a young boy that doesn't understand his ability and is terrified by it. (The twist he found was awesome, but to me it was more important to wonder "What is a Medium's life like at nine years old?".)
Your best chance of commercial success is being original.
Your best chance of being original is writing what you enjoy.