I find with any phone, none are the best. I use a selection to get a reasonable forecast. That said, UK based ones tend to be based on Met Office. Currently BBC is based on it but that’s due to change having scrapped their contract. US based apps are too generic on UK as a whole, and often don’t have a fine enough location. E.g, nearest to places in South East may just be London.
Best as in fluffy features and widgets, not interested myself. I just want a decent forecast. Don’t want anything that hammers the phone to show something fancy looking.
However aside from forecast, I recommend Rain Alarm. Not a forecast but shows you current reality as in where it actually is raining, and gives you alerts to nearby rain.