A D20 with one 1 and nineteen 20s. For when the party's plan should probably work... but you might as well check, right?

Full list in this set: D20 D20 - Large Dice Set Dice Set - Large

All "standard size" dice are scaled to match a 22mm D20 (measured from opposite corners), and all "large" dice are likewise scaled to a 30mm D20.

See also: All Ones D20 - Large All Twenties D20 - Large One Twenty D20 - Large Coinflip D20 - Large