Ok, I would go with
Isabella and
Sophia - yes, they are WAY too popular, however, there is no denying they aren't pretty names, and they do sound good together because they have a similar 'feel' to them.
Olivia and
SophiaIsabella and
Olivia
Anna and
William - so classic, I adore it
James and
JuliaOlivia and
William
Alexander and
BenjaminJames and
William - clear winner for me