I did the top 4 (Maths, Biology, Chemistry and Physics) and really enjoyed all of them.
Maths by far has the least content out of the lot, but is really interesting at the same time, especially if your school lets you pick the applied modules that you do. Chemistry was my favourite as the theory is fun to learn and the practicals break up lessons so it's not like you're constantly writing. Biology has the most content and is more about rote learning than comprehension in places but then there are areas which are really interesting, I personally really enjoyed the topics on DNA replication and respiration. Physics was the biggest jump from GCSE to A Level, with the first module being really mathematical but then there's lots of qualitiative stuff on quantum theory that you get to learn.
Do you have any idea what you'd like to do at university?