There is no single best platform. The right choice depends on where your readers are, what you want to control and what each service pays for your book.
Where do your readers buy?
If most readers use Amazon, KDP is the obvious start. If you want bookshops and libraries to be able to order your book, look at IngramSpark. Barnes & Noble Press reaches that retailer's customers, and Lulu offers direct sales and flexible formats.
What does each pay per copy?
This is a calculation, not a guess. Enter your page count, trim size and list price into the royalty calculator and compare the figures side by side. Remember that some platforms apply a wholesale discount before they pay you, and printing costs vary, so the same price earns different amounts.
How much control do you want?
Using your own ISBN lets you choose your imprint and keep the same edition across platforms. Platform-supplied ISBNs are convenient but may tie that edition to the platform. Decide this before you publish, because changing it later means a new edition.
How much work will you accept?
Each platform has its own file checks, cover template and dashboard. Two platforms double that work. Start with one, get it right, then add another when your routine is stable. The sales tracking guide keeps the reports manageable.
Compare, then confirm
Use our platform rules page for the figures and the links to the official sources, and confirm in the platform before you publish. Our platform comparison article gives more background.