The artifact you control has to have the highest mana value only among artifacts on the battlefield, not among all permanents on the battlefield.
2016-09-20 • wotc
If an artifact has {X} in its mana cost, X is considered to be 0.
2016-09-20 • wotc
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 • wotc
If you no longer control an artifact with the highest mana value as Padeem's triggered ability resolves, you don't draw a card.