I like Due. I’ve been using it since 2011 and it’s been very reliable......I even use it as my daily pill reminder, though for me, missing a dose wouldn’t be fatal, and I’m certainly not recommending it specifically be used for that just in case it does fail to alert you.
It can repeat alerts up to 10 times at intervals of between 5 minutes and 1 hour. According to the app, 10 times is a limitation of iOS, but it can repeat them indefinitely if the app is opened, or you interact with a notification from the app.
It’s fairly no frills, lacking things like multiple lists, but it does exactly what it promises to do. It also has the option to snooze reminders for varying amounts of time from the notifications.
Here’s a link to the App Store:
Due - Reminders & Timers by Due Apps LLP
https://itunes.apple.com/gb/app/due-reminders-timers/id390017969?mt=8