Again, the answer is Mind Twist, and its not close
But they wont last as long as a good Ultimate Guard, Dragon Shield, or other brands
The current mulligan rule for most formats is the London Mulligan, where you always redraw a new hand of seven cards regardless of how many mulligans youre up to, but then return cards from that seven to the bottom of your library until you reach the size youre meant to be at
Mark Rosewater has explained that they exist to balance the metagame and to encourage players to play multicolor decks
In my example, Y'shtola, Night's Blessed is a great commander to maximize my use of Toxic Deluge, so in the Deck Filters area, I started typing the card name, it popped up so I selected the card and clicked the + button to the right and now the green box below it tells me the decks I view will include Toxic Deluge