biologists and geologists mostly, of a variety of subdisciplines but no one too specialized at the same time, you want people who know how to take samples correctly and how to assess comparative value of samples, and can work with may aspects of the field equally well.
Like Nasa missions there will be a lot of cross training, you are never just an expert in one field. The more expertise you can pack in each brain the better.
That said overspecialization makes you useless. This is especially true for biologists since right now we only have earth life to study. Your scientists need a broad base. A botanist for instance would be less useful than a general biologist (of equal quality) since whatever is fulfilling the plant like niches is unrelated to earth plants and thus will not mesh well with the assumptions a specialist is used to working with. Likewise a geneticist will be useless if the life there does not use nucleic acids, but would be very helpful if it does, so you should decide if they go in with any information.
This is also affected by how much room you have, and if you know anything about the planet before you go, if you already know it has life for instance biologists become a lot more important and you will want many of them.
Exo-geologists have to understand a lot about many different aspects of geology since many of the familiar rules of geology change on different planets. So you want at least one geochemist, and one exo-geologist at bare minimum. geology is less troubled by specialization since, for instance, volcanoes are volcanoes no matter which planet they are on, so if you have space for a volcanologist bring one.
biochemists, these are the people who will be doing the initial analysis of many samples so you you are going for an extended visit you want some along. for short visit they are less necessary, although would be a helpful bonus if there is space, and a must have for cross-training.
Paleontologists,mostly as cross training, that gives them an advantage with dealing with unexpected ecosystems, and you really want someone looking for fossils.
A Planetary climatologist although for the most part they can work remotely provided you have decent satellites, but if you have room or are staying for a while definitely bring one, they will end up working with the geologists a lot. Even for a shorter visit you definitely want some cross training for this to take some ice cores if possible.
If the planet has oceans, a oceanologist would be a must have, with the same specialization caveat as the biologists.
A medical expert will be important not just for normal mission needs but also for understanding how the world affects your explorers, this is especially important for longer missions.
Ideally you want to release a lot of satellites and remote drones/propes so you cover a lot of land. Even more ideally you want satellite map before you even plan on landing, so you know where the most interesting looking things are. No expedition would be sent to a planet that had not been heavily observed by remote/satellite means or without a map of at least the majority of the planet's surface.
Note I am assuming we are talking about people on the landing mission, there may well be other scientists in a orbital ship, ones who are not concerned with the planets surface, of who can work better remotely.