In a nutshell, the most popular ones if you have up to 20 units are Cozy, Okupied, AppFolio, TenantCloud. If you need just rent facilitation, then Cozy, as well as Zelle, Venmo or other free tools can work well as they are free. However, with them, you will still need to manage separate spreadsheets or another app for expenses, lease renewals, etc.
If you want to not only collect rent but track expenses, generate IRS reports, and get phone reminders for events, then Okupied is your best bet. We have been using it for a year now and it is very user friendly.
If you want additional features such as tenant screening, maintenance requests, documents upload and e-signatures, and a bunch of other features, then AppFolio and TenantCloud could work, but they are not user friendly at all.
So it the end, it all depends what you need accomplished besides rent collection. I am always on the go, so for me a mobile app works great, but some people like the convenience of desktop applications. My suggestion is create a free account and just test them out to see which one you are most comfortable with and is easier to use.