Complicated question....
There is an enormous amount of engineering that goes into the optics of a headlight lens and reflector. The best thing, is what it was designed to use, a genuine Subaru bulb. Make sure they are OEM assemblies, and the lenses are nice and clear.
There are HID drop-in bulbs. But an HID "filament" is a long arc, and will cast light from many angles that was never intended. This pretty much always leads to light scattered all over the place....
There are LED drop-in "bulbs". On LEDs, the light originates from just a point, smaller than a halogen filament. This means you actually get a little less coverage (I have these for low beams in 2 of our Outbacks, and find it to be tolerable, but I still use halogens for High beam). The vast majority of LEDS are 6500k, which is more on the blue side of the color spectrum, which is not ideal for snow.
There are many people who have retrofitted HID projectors into the factory housing. Not a small job, and must be done with a high degree of accuracy for correct performance.
Some people swap for headlight housings from other markets. But keep in mind, the optics are not symmetrical, using RHD lenses and then driving on the RH side of the road is bad juju.