There's an argument for all of them being free but if I had to pick then the ones that give back like doctors and teachers, and maybe with a caveat such as in order to study this degree for free you need to get the grades for course entry, if the government is paying for accommodation they should live within their means, maintain grades at or above a certain level, and the student has to go into the relevant work field for a certain length of time. The textbooks should be free, too, with the course. Not sure what should be done about other stuff but government could also fund accommodation to an extent? Maybe not pay for everything, but government housing with no rent, and then they work to pay the bills and food and other stuff?
So, for example, (and this is just an example ft numbers I pulled out of the air)
if someone wanted to study to be a teacher for free they first need the grades required for their chosen course at their chosen uni, they would need to pick accommodation that is bare minimum rather than extravagant (like if it's just them then a basic studio flat, but if there's also a sibling/child they look after a 1 bedroom with sofa bed or something. IDK I'm talking bare minimum here), they should keep their grades at a level worth 2:2 or above, and they should do work experience during their course in teaching and have worked as a teacher for at least 5 tax years within a decade of graduating. The books for their course should be free along with the actual course, and working a part time job on weekends should cover the bills, food, transport, while still leaving study time.