I suspect that the biggest selling US Motown single released in 1979 was actually Smokey Robinson's "Cruisin'" rather than The Commodores "Still".
The record was a real slow burner and actually topped the Cashbox Pop chart for one week on the 16th February 1980, though Billboard never listed it higher than #4 on both the Hot-100 and the R&B chart. For reference "Still" spent one week topping Billboard's Hot 100 on 17th November 1979 and topped their R&B chart for one week on 24th November 1979.
In terms of chart longevity "Cruisin" won easily, 26 weeks on The Cashbox chart ("Still" lasted only 19 weeks), 25 weeks in Billboard's Hot-100 ("Still" lasted 20) and by a phenomenal 28 weeks versus 17 on the R&B listing.
If only there were reliable sales figures available for that era.

Roger