If we count using miles as a proxy, then I may have the best one-way deal:
Applied for Barclays/AAdvantage Aviator Red Mastercard in July, deal was waived annual fee for a year, spend anything on the card in 3 months and get 60500 AAdvantage miles
Paid for a $10 lunch and didn't use the card for anything else before the miles credited
Booked SFO-JFK-BOS in A321T F for 38500 miles + $5.60
2773 miles, $15.60
$0.000405/mi
$0.0405/100 mi
They were selling A321T J for 24500 miles + $5.60 as well.