field1 = forms.ModelChoiceField(queryset=...)

class YourForm(forms.Form):
    field1 = forms.ModelChoiceField(queryset = MyModel.objects.all() )

form = YourForm(initial = {'field1': instance_of_mymodel.pk })

form = YourForm(initial = {'field1': instance_of_mymodel })

field1 = forms.ModelChoiceField(queryset=..., initial=0)

form = YourForm(initial = {'field1': instance_of_mymodel })

form = YourForm(initial = {'field1': instance_of_mymodel.pk })

form = YourForm(initial = {'field1': instance_of_mymodel })

field1 = forms.ModelChoiceField(queryset=..., initial=0)

field1 = forms.ModelChoiceField(queryset=Model.objects.all(), empty_label="Selected value")

form = YourForm(initial = {'field1': pk })