The 401 at it's busiest point, just west of the Highway 400 interchange, handles 458,000 vehicles daily during summer months (
Source, page 1074). I'm sure the summer weekday number is well about 500,000, since the 458k number averages in weekends which have lighter traffic flows.
This is known to be busier than any highway in the US or Europe, and China doesn't really have any roads large enough (14+ lanes) that could handle that level of throughput, no matter how congested.
The only other real possible contestant to "beat" the 401 is the Marginal Tietê in Sao Paulo.. and while it may be busier, it has a 4 lane "collector" lane that has at grade intersections on it that technically disqualifies those lanes as freeway lanes, as well as any traffic on those lanes as "freeway" traffic. I would be surprised if the remaining lanes in the two "express" carriageways amounted to more traffic at any given point than the 401.
The 401 probably is the busiest freeway in the world, but mostly on a technicality.