Airalo is one of the best-known providers and, at the same time, one of the most transparent; it operates in the mid-price range. With its unlimited plans, you typically get 3 GB of high-speed data per day; after that, speeds drop to 1 Mbit/s.
Airalo operates worldwide and is considered very reliable. You can sign up for unlimited plans for a specific number of days or for a fixed data allowance. There are plans for individual countries as well as regional plans.