According to Nielsen IAG, an Dean asking Sam about the Apple iPod hooked up to a jack in their car in the September 18 episode of Supernatural was the most recalled product placement in a broadcast network scripted series for the period between September 15 and October 12.
Other TVbytheNumbers fan favorites Chuck and Terminator: The Sarah Connor Chronicles also made the top ten most recalled product placements list. Chuck at number six with the Crown Victoria episode on 9/29, and TSCC at number ten with the Dodge Ram pickup on 10/6.
This isn't measuring total exposure of the product placement, rather the percentage of viewers who can recall the product placement within a 24 hour period. The iPod placement in Supernatural was recalled 2.3 times more than the average product placement. Even more than a month after it aired, I remembered the iPod scene in Supernatural.
You can see the full top ten list on the NielsenWire blog with an explanation of how the index is calculated.