You may cast any number of spells from among them without paying their mana costs
Whenever new cards are added, new combinations rise up, new ideas and team-ups reveal themselves, and the game changes, ever so slightly, to expand the fun and strategy
Mutate is similar
Being able to make non-legendary copies is always great for any MTG commander that you'd like to have two copies of, and in a fliers deck or something, your board will get very big very quickly
Liliana of the Veil As a discard tool Liliana is kind of weak yet she remains atop this list because the function of the card as a whole is so rounded and effective
The flexibility of an artifact creature at the kitchen table, particularly, with respect to colorless ones, is pretty important to evaluating it as a card