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%.
