That ability lets you pick once from four different effects: tapping or untapping a permanent, dealing 3 damage to each opponent, copying an instant or sorcery, or (presumably your last choice) putting Gandalf back on top of your deck
I snagged The Abyss in pack two, which notably does not affect artifact creatures, and in the third pack I actually took Tinker over Mox Emerald because Tinker was more likely to lead to a broken start in my deck at that point, and had significantly more value later in the game
Theres no single best MTG card, since there are a lot of contexts in which a card can be powerful
It will be available via lottery starting in February
Where to Start Playing Magic Online MTG: Online Magic: The Gathering Online has been around for years, and is a pretty good translation of in-person play to this online client
This time around, they grant the Monarch and have differing effects based on whether you retain it through to your upkeep (just like Commander Legends Court cycle)