There is no "best". It all depends on what you are doing and what look you want. Plus X Pan was
appropriately marketed mainly as a studio portrait film. It had a very long sweeping toe designed to
give subtle midtones and highlight gradation. If you had a soft lighting ratio you would get gradation
in the shadows too. But with strong lighting you'd have to resort to compensating dev to dig deep,
and that would likely spoil the delicacy of the uppers. Not a very good film for direct sunlight conditions with deep shadows. The closest thing today in curve shape is Delta 100. FP4 has a much longer straight line, as does ACROS, though neither are true straight line films.