I always go for
Margaret. The only issue I've got with it is that where I live, the default nn is
Margie, with a hard g rather than a j. I like both
Maggie and
Peggy, though I really don't think
Margaret needs a nn.
Marguerite seems silly if the family isn't at least somewhat French, and
Margot has never appealed to me, with its air of suburban snobbery.