My guides

Super Magical Chest!

The super magical chest is the one you want to buy from the store if money is not an issue. Buying this chest gives you the most epics and rares for the gems you spend. At arena 8 the chest costs 4600 gems however, which is around $50, so it is not a cheap chest to buy at all.

The super magical chest have a very high chance of containing legendary cards. From research that various YouTubers and other players have done, it seems that the chance of getting a legendary card from a super magical chest is around 33%.

You can get a super magical chest from winning battle as well, but the chance is very small. I would estimate the chance to be less than 0.2%. I have only gotten one chest from winning over 1000 battles in Clash Royale. When you get the super magical chest from winning battles it will just replace on of the other chests in the drop cycle.