Imgur is popular, but there are a great many sites out there so do a bit of reading to find one that suits you…
[​Finding a backup site is also recommended, even if it doesn;t work as well for posting. (See the comments about the recent changes to Discord image hosting.)]

(One thing to look out for is that many sites will permit 'adult' images and may show similarly themed ads, whereas imgur has a quite strict policy against this.)