Personally, I think best of all time is the Shields system from Duel Master.

Each player puts five cards from the top of their deck face-down at the start of their game; these are their Shields. Whenever a player takes damage, one of their Shields breaks and goes into their hand. Many cards would have “Shield Trigger” effects that would allow them to be played after entering your hand this way. Some, such as Holy Awe, could turn the tide of the match.