Artifacts you control have hexproof. At the beginning of your upkeep, if you control the artifact with the highest mana value or tied for the highest mana value, draw a card.
2016-09-20
If an artifact has {X} in its mana cost, X is considered to be 0.
2016-09-20
If you don't control an artifact with the highest mana value as your upkeep begins, Padeem's second ability won't trigger. You can't cast an instant to destroy an opponent's artifact during your turn before your upkeep begins.
2016-09-20
If you no longer control an artifact with the highest mana value as Padeem's triggered ability resolves, you don't draw a card.
2016-09-20
The artifact you control has to have the highest mana value only among artifacts on the battlefield, not among all permanents on the battlefield.