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.
The best card game I’ve played is Innovation.
Cards contain different icons, which indicate supremacy for the player who is displaying the most of that icon on their tableau.
Supremacy means that a player can demand things of players who are showing less icons.
The standout card-mechanism is when you perform an action to ‘splay’ one of your stacks of cards, thus revealing extra icons from the cards below.
An elegant and exceptional game.
Highly recommended to try.
Available to play online, with a free account, on yucata.de:
https://www.yucata.de/en/GameInfo/Innovation
Review here:
https://islaythedragon.com/game-reviews/sprint-through-the-ages-a-review-of-innovation/