I use Due. It lets you set recurring reminders and has some really useful auto-snooze options. I think the free version lets you auto-snooze for either one minute or one hour, but the paid version lets you choose 5, 10, 15, or 30 minutes, etc. I really like this feature because if something is urgent I can set it to keep nagging me every 5 minutes… or even every minute. If it's not so urgent I can set the auto-snooze for a longer period. When you get the reminder on your watch you can tap to mark done or reschedule. If you ignore or dismiss the reminder, it will remind you again at the next auto-snooze interval. It's easy to add reminders in the iPhone app, or you can use "Hey Siri, remind me to…" and Due will automatically set the reminder for you. Due also lets you set up custom timers you can access from the watch app. This has been one of my most useful (and frequently used) watch apps.
My calendar app (Fantastical 2) also has a reminders feature that supports "Hey Siri," and my task manager (currently TickTick) offers reminders for tasks. Then there is also the stock Reminders app for iOS, so a dedicated reminder app isn't necessary for me. However I like using Due enough to keep using it for reminders. I guess I just prefer a separate reminders list (which tends to be very short) for things I absolutely need to be nagged about. My task and project lists are much longer, and I don't usually set reminders for them.