I’d say a sunny day easily, but it depends on what there is to absorb the sunlight. Snow is a great insulator and reflects plenty of light, but if there’s Even a few specks of dirt on the snow they will condense and melt the snow faster. Same with anything that absorbs light. If it’s a barren Arctic desert with only snow, then the temperature would be more important.
This is based on observation, I’m no scientist until a peer reviews this
Which will melt more snow: 35 F and sunny or 45 F and cloudy? (kbin.social)
Related question: What cloudy temperature will melt as much as 35 F and sunny?