What was the lowest ground speed landing?
Considering groundspeed as a vector, 0.  

You can land vertically with strong headwind: Could a plane land vertically in a strong headwind?

As the answers to the question, previously linked in Flexo's comment -  Is it possible to fly backward if you have really strong headwind? - detail, the answer is "yes". It's even possible to have one's groundspeed vector directed tailwise. 

The Antonov An-2, can be flown at as little as 25 knots of airspeed, and safely landed below that speed. When turned into decent headwind, it can be kept in hover relative to the ground. I haven't found a video of a hover landing, but it's been reported to be done. With a 12-passenger capacity, it's no 737, but it can count as a passenger airplane, and it's designed to do this.

Vertical landings of smaller planes have been filmed: 
                
             (thanks bukwyrm).

P.S. Vector representation is most appropriate for groundspeed, as it's not normally directed along the aircraft's longitudinal axis due to side winds. So landing "backwards" can be considered positive groundspeed once again.