7.3, for sure. Argus was about as close to a mini-expansion as they could get in terms of patch content, and it wasn't overshadowed by BFA the way 7.3 is being overshadowed by Shadowlands, no pun intended.
But that's also really not the whole picture.
A more accurate comparison would probably be 7.3 (Argus) to 8.2 (Nazjatar + Mechagon), 7.2 (Broken Shore) to 8.1 (Darkshore), and 7.1 (Legion Assaults) to 8.3 (Black Empire Assaults).
It's roughly the same amount of content, just with different cadence and timing. Actually, BFA may even have more conent overall when you factor in Islands and Faction Assaults.
The real issue is that Legion just had a more compelling narrative and more engaging gameplay.
And in retrospect, maybe the story should have flowed a bit differently. Mechagon and naga assaults in 8.2 with Eternal Palace raid, culminate in Nazjatar being raised from the deeps and War Campaign finale, 8.3 is Nazjatar, Black Empire assaults, and Ny'alotha (entrance in Nazjatar).
Roughly the same content overall, but 8.2 is smaller and 8.3 is way larger with a much longer tail leading into 9.0.