The answer to the best invoicing software depends upon your perspective; What do you think is best?
But it is much related to your main goal and the side B to it. (of course, every story has some side B)
If you are a freelancer or a small business trying to find a solution to manage your accounts but do not want to involve any other person to do the job, there are plenty of apps that can make your work possible. Some of these are completely free and provide free invoice generation with complete account management at teeny service cost. You don’t have to pay a penny once you’ve subscribed. But if you expect these invoicing applications to be completely free, it’s a bit too much because quality always comes with some investment, and that’s the first rule of business.
But the good thing is, some of them are providing their services with minuscule product charges.
One of these Invoicing Software is Invoicelabs. It is an invoice generation and bookkeeping software that handles the invoicing needs of small-scale businesses and the freelance community pretty well. It is available as an android application along with a web-based platform. What I like about Invoicelabs is the freedom to work on the billing software according to my ease, and the perfect thing is, all the data gets completely synchronized on the mobile application or any device I use.
Some of the services it provides are:
Unlimited invoice generation
Customizable invoices
Invoice estimation
Multiple customer support
Inventory management
Secure cloud storage
Invoice tracking
And of course, if something is working for one person, it may not work for the other you have to test it yourself. If you want to try this one, you can easily download it on play-store, and in your journey, if you find some better, do tell me too☺

https://play.google.com/store/apps/details?id=com.invoiceslab.invoice&hl=en_US&gl=US&showAllReviews=true