Best collection of puzzles?

I am looking for some good collection of puzzles (primarily tactical ones), preferably in digital form, preferably free. I don't care if it is in an app, a website, a pgn or whatever, as long as I can casually solve puzzles on my phone. If it can track my progress or even choose puzzles for me based on my strengths and weaknesses, even better. I am not impressed by chess.com's puzzles, too many of them have more than one solution where only one is marked as correct.

Please post an example of a chess.com puzzle that you believe has more than one solution.

You just joined. You haven't even tried any puzzles.

@justbefair I don't believe in OP's statement that most chess.com puzzles have multiple correct solutions too, but I've seen one recently that I believe should be changed. The final two moves are not necessary, simply trading the two pieces for a rook still leads to a completely winning pawns endgame. 

https://www.chess.com/puzzles/problem/1129506

I got this,...

https://www.chess.com/announcements/view/the-ultimate-collection-of-intriguing-puzzles

I'm planning on doing one on studies soon

Note that these puzzles are for pure enjoyment, and are not meant for excessive solving.

Chess: 5334 Problems, Combinations and Games - László Polgár

People speak very highly about the tactical richness of ChessTempo.com 

My personal favourites are the courses by https://chesskingtraining.com/

You have to pay for them but they don't cost much.

chess king has dozens of apps for mobile phone. Each contains a set of free puzzles. 

The 1001 series on chessable.