Android

You can ask this and probably get a different answer for each comment! It really depends on what you want from it and what data sources work best where you live. I'll mention a few:

• Forecaster is super simple, displaying the forecast for each day as a bar. Yellow represents clear weather, grey represents cloud, and so on. I quite like it, although you can only see weather, temperature and wind, and it uses Apple Weather with no option to use something else.
• Clyma is by far my favourite. It looks the best and offers the most data (like wind gust and feels like temperature) and displays it in a graph. Although it's very unreliable, and sometimes stops refreshing entirely. I have to clear the app's data, then enable the location permission before I launch it again to get it working. So I can't rely on it that much.
• Overdrop is a little bit like Clyma but it hasn't made it onto my new phone.
• Today should get a mention. I don't use it much but it's got by far the biggest array of sources to choose from, and you get data on air quality that you don't see much elsewhere.
• Meteogram is great if you really like customising and getting into the data itself. You can completely change how the graph looks and what data is included. I bought it on sale a while back and still have access to the old "Credits" system, so I can't speak for how the new subscription model works but there's still a free tier.
• Forecastie and OpenWeather are nice ways to access OpenWeatherMap data if that's what you prefer.
• The Met Office app is a nice way to get notified of warnings if you live in the UK.
• And RainViewer is a very nice radar app, but the imagery doesn't seem to update very often here (even with Premium, which is meant to give you a 5-minute resolution. Other apps have this imagery, so I don't quite know why they don't because they definitely used to)

Hopefully some of that helped someone!