Anki. End of discussion. If you don't know what Anki is, it's a flashcard app that automatically schedules cards for you based on how hard or easy you say the cards are. So if you tell Anki a card is easy, it might, say, schedule the next review for 30 days later, whereas if you say it's hard, it might schedule the next review for 2 days later. Anki:
- Is free and open source
- Has a huge community, and with it, tons of plugins, premade flashcard decks, tutorials, and integration with other programs
- Just has tons of features and customizability in general.
- Just recently got a new scheduling algorithm called FSRS, which is scientifically proven to be one of the most accurate flashcard scheduling algorithms in the world at predicting when you're on the verge of forgetting a card (but make sure you enable it in the settings)
- Has synchronization between mobile and PC
Anki can be a bit intimidating, as there's a bit of a learning curve to using the app in the first place, but I promise you that all the other options like Quizlet or whatever aren't even close to as good as Anki.
I mean, pretty much the whole appeal of modern Yu-Gi-Oh is that you go ridiculously fast and set up crazy convoluted combo lines. A lot of people get mad that modern Yu-Gi-Oh isn't the same game it was when they were kids, but what can I say, the game is more than 25 years old and has changed a lot in that time.