If you are aiming "just" to become a generic scientist, then hard work is definitely more important, unless you are going for math or physics, then you need a bit of intelligence as well. But otherwise, just hard work and persistence.
If you are aiming to become an "all-time great" of a scientist, then you both intelligence and hard work, but you also need creativity and courage. You need the creativity to think outside of the already established box, and the courage to express those thoughts among your peers, come what may.>If you're a 120 IQ brainlet you might as well become a janitor because all real science requires an IQ 140+
No. You can easily get a PhD with 110 IQ. You can even get one with 100 IQ, and possibly as low as with 90 IQ, but it will just require exponentially more work and discipline.