For me it has to be propulsion because chemical rocketry is very expensive and very limiting. There are two aspects to the problem. Lifting stuff into earth orbit and then achieving viable velocities for interplanetary travel, and later interstellar travel.
Of the two aspects of the problem, lifting mass from earth's surface into orbit is at present both difficult and expensive. The chemical rockets used have to carry their own fuel which makes up a considerable proportion of their mass, leaving little left for payload. Sometimes they have the inconsiderate habit of self destructing destroying the payload, which is highly inconvenient. Unfortunately until we figure out an alternative way of inserting material into orbit we are stuck with rocketry.
There have been differing suggestions over the years for inserting material into orbit with some being rather exotic, such as the space elevator. However I believe that there are significant physics problems to be addressed before this could be considered. The problems don't involve exotic particles, nuclear forces, or quantum physics, but basic Newtonian classic physics - gravity and rotational forces, such as the coriolis force, because the elevator would be firmly anchored to the earth's surface. Then there are the engineering problems.
The second aspect is drive packs. What form do they take? What can we look at technically achieving in the next 50 years? Do we need to start prospecting for dilithium crystals?

Any drive packs that we use are going to have to be safe and have shielding for any hazardous emissions. However the last thing we need is having to hump tonnes of lead around space. It takes up room and has to be lifted into orbit.
So many questions to be asked and answered.