Melody Anne is basically the queen of billionaire romance and family sagas. Her most popular series is definitely the 'Billionaire Bachelors' series, which is a massive, interconnected world. I think 'Luca' and 'Arianna' are the most-read titles because they kick off the whole thing and get recommended as the entry point by everyone. The 'Anderson' series is also huge; it's a bit more family drama mixed with the romance, and I see 'Jake' and 'Joseph' getting talked about a lot on romance forums.
Honestly, her fans devour these books because they're predictable in the best way—you know you're getting a steamy, dramatic, feel-good story with a guaranteed happy ending. They're comfort reads. I find the sheer number of books in these series a bit overwhelming, but the popular ones are always the first few in each series because they set up the whole universe.
For pure popularity, I'd say start with 'Luca' from the Billionaire Bachelors. It's the blueprint for her whole style.