When it comes to self-publishing, it really depends on what you're looking for and what platform you want to use. If you want to get a book out quickly, Amazon Kindle Direct Publishing (KDP) is a good option since it lets you upload your book without any additional formatting and it has good royalty rates. It's for e-books only, though, so if you're looking for print books, you'll need to use a different platform.
For print books, you might want to look into Blurb, which is a popular self-publishing platform that allows you to select from dozens of book sizes and layouts. They also offer helpful resources and technical support if you need it.
There are other options available, too, like Lulu and IngramSpark, and they all have different advantages and features. It's best to do your research to determine which one suits your needs best. Good luck!