As for i know, one piece's sales in US is around 3-4 million. Which i considered very low but now knowing best selling numbers they are decent numbers in comparison. They are low from naruto by half ( per volume numbers are more lower than half though) but considering what i always hear that one piece is not very popular in US they are still good numbers.From the little data I can find, Naruto is first, and by far.
This would be BookScan data (so sold-through):
Naruto would be around 7M+ units sold by now. (6M+ as of 2018)
Dragon Ball would be around 3M+ (2M+ as of 2018).
This would mean that My Hero Academia beat Dragon Ball in 2021, and is on pace to beat Naruto within the next few years.
Something like Fruits Basket could be above Dragon Ball.
Demon Slayer too, if we consider what it likely sold so far in 2022.