When Blitzcrank pulls an opponent's unit to my battlefield, who is the attacker and who is the defender?

Ruling: When Blitzcrank pulls an opponent's unit to your battlefield, the pulled unit's controller becomes the attacker and you become the defender.

Sequence:

• Blitzcrank enters your battlefield (which you already control)
• Blitzcrank's "When you play me" trigger activates
• The trigger moves an opponent's unit to your battlefield
• The pulled enemy unit enters a battlefield controlled by a different player, applying contested status
• The pulled unit's controller becomes the attacker; you become the defender
• Combat stages and resolves

Nuances:

• The attacker is determined by who applied contested status to the battlefield
• Since Blitzcrank was played to a battlefield you already control, it does not apply contested
• The enemy unit being moved to your battlefield is what applies contested status, making its controller the attacker