If you no longer control an artifact with the highest mana value as Padeem's triggered ability resolves, you don't draw a card.

The artifact you control has to have the highest mana value only among artifacts on the battlefield, not among all permanents on the battlefield.

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.

If an artifact has in its mana cost, X is considered to be 0.