TLDR: If you are looking for a 'classic' solitaire game without weird gimmicks, this is the game for you.

This is most likely the largest collection of solitaire variants anywhere. At the time of writing this, there are 200 variants in the game. Many of them are, of course, slight variations of each other, but none are exactly the same. You can look up the rules for each variant either in the menu or even while in a game. Very convenient.

All variants are available for play right from the start in "free play" mode. The difficulty varies in several steps from "very easy" to "hopeless". You can also "undo" every move you made or get a hint for a possible move. If you don't want to move the cards manually to the foundations "autoplay" is your friend.

If you care about customization options (I don't):
You can unlock different backrounds, decks and card backs while you play. The more you play, the more you unlock. I'm perfectly fine with green felt and standard card decks though, thank you very much.

There is also an "expedition" mode which I actually never really cared for. If you played something like this before, maybe this is a bonus for you. It can be completely ignored though without missing out on anything of importance.

If somehow your favourite variant is missing from the game (I doubt it) feel free to ask for it on the forums. The developer is very active, responds quickly and will more than likely try to add it in a future update.

Pro tip if you get frustrated with the higher difficulties: There is a thread in the forums where people provide winnable seeds for the "hopeless" and many "very hard" or "hard" variants. If you use these seeds, the game at least provides a theoretically winnable board to begin with. Good luck!