Well, there is some bias here haha, but the best free tactics are on The Exchange. There is no paywall for puzzles (or any part of the app) and the puzzles are the most game-realistic possible (since they come from real games with explanations shared by real chess players). On top of tactics, the app provides an easy way to share annotated games and learn from others. The more the merrier on the app, so would love to have you join (searchable as "The Exchange | Chess" on the App Store and Google Play)
If you want a less bias answer, and the ChessTactics mobile app are my next choices.
Does anyone know of any good puzzle apps? I want to practice more and train tactics, but I don't want to pay for Chess.com and I struggle with seeing puzzles in games anyways... Any suggestions?