It depends. Some websites have legal licenses to offer manga for online reading, but many others do not. Reading on unauthorized sites is usually illegal.
Whether reading manga online is illegal varies. Legitimate platforms with proper licensing are fine. However, sites that illegally upload and distribute manga without permission are breaking the law, and reading from such places is considered illegal too.
For me, MangaDex is great. It offers a good user interface and frequent updates. However, it's important to note that the availability of manga can vary depending on licensing and legal issues.