Anki
Pick it if you drill large decks, want the deepest scheduling and add-ons, or study from a shared deck such as a medical one.
Anki is a free, open-source flashcard app built around a spaced-repetition scheduler. You make the cards or import a shared deck, and Anki decides when each one comes back.
Strengths
- Deep scheduling with FSRS and a very large add-on and shared-deck community
- Free and open source on desktop and Android, and it works offline
- Built-in image occlusion and full control over card layout
Limits
- Steeper learning curve and a plain interface
- You write cards by hand or import decks, so building a deck takes time
