Let's start with an interesting ranking, namely the ranking of the best slots to do bonus wagers. Many casinos as you well know, offer bonuses but they have to be wagered.
How do bonus wagers work?
I'll explain by giving you a practical example.
Let's pretend that casino "A" gives you a 100% bonus up to a maximum of 100€/$.
You then deposit 50€/$ and the casino adds another 50€/$ bonus to you. So far so simple. Problem: It's not that you immediately withdraw the 100€/$, because otherwise the casino goes broke right away.
In order to convert the bonus into real money and then withdraw, you must first wager the bonus, usually 35 or 40 times the value of the deposit (depends on the terms and conditions, which is why I always encourage you to check).
Now... it means that following the example above, you would first have to make a wager of 2000€ before you can withdraw. Clearly this also means that you need slots that pay, otherwise with 100€ make 2000€, it's not really easy.
Therefore, here is my personal ranking of the best slots to make bonus wagers: