Full Term is probably the most commonly recommended one, it tracks both the duration and the gap between contractions, and people find it really straightforward to use without having to figure anything out mid-labour. If you want something more minimal, Storky is another free option that does the basics well, though some mums feel it's a bit bare-bones. My friend used Gentle Birth and liked that it had a free contraction timer bundled with a hypnobirthing voice guide, which she said actually helped her stay calm between contractions. Honestly, all three are free, so it might be worth downloading Full Term and Gentle Birth to see which layout feels more comfortable to you before your due date.