You are not allowed to ask "what's best", because what is best depends on many things!
Anyway, with "--partitions" you only specify which partitions x264 will consider at all (and more of them takes more time). Still, x264 will pick the "most suitable" partition - from those that are under consideration.
So enabling "p4x4"
in addition certainly shouldn't hurt, though the benefit may be relatively small. Which means it may
not actually be worth the extra computation time.
If you just want the best possible compression (and encoding time doesn't matter that much), you can pick "--partitions all" and that's it. Apart from that, I recommend just sticking with the "--preset" system.