I am using Flo and I absolutely love it. And there are many reasons for this:
In fact, I can write many more pluses, but you'd better find out about them yourself. It's better to see once)
I use an application with a simple and understandable name "Women's Calendar".
From the functionality: 1 - the calendar itself, which is displayed in the usual format with additional information for each day about the chance of pregnancy. The week before ovulation is marked with special icons for special vigilance :)
2 - History. This section essentially collects statistics on previous periods, showing their duration + cycle duration.
In general, nothing superfluous: minimal advertising and ease of use.
I use the Clue program, of all that I have tried, this one is the most convenient. It is almost completely customizable, intuitive to use, clear and pleasant design. Sends good and timely notifications. With its help, you can track your periods, remember all your feelings, write down when there was sex and whether it was protected, enter any examinations, attach a note, and so on. As soon as I started using Clue, I removed all such apps from my phone.