Maths, chem and bio/physics
Pros:
(I did Chem and Bio)- Bio and Chem go together really well, doing maths may help with the statistics you do in biology.
Will get you onto a vast number of STEM courses, particularly environmental, medical, biochemical stuff etc.
Physics equally goes very well with Maths and does cross over with Chem. Great if you wanted to go down the more technological side of STEM later on.
Cons:
Maths and Chemistry are, by themselves, incredibly challenging. Out of the three I did (History, Chem, Bio), Chem was the hardest by far. I found Biology intense because of the sheer amount of content. I found History pretty chill.
May not prepare you for essay-writing and in-depth independent research as much.
Maths, chem and art subject (e.g. English/History)
Pros:
English is challenging at A-Level but from people I know who did STEM subjects with English, they found English easier. I also, as aforementioned, found History a lot easier.
Gives you a spread- English and History really helps you learn to write properly and appropriately.
Also gives you experience with having to do *a lot* of independent reading.
Cons:
May limit the scope of what you can study in the STEM field
Have to really enjoy English/History as you study the same topics for long periods of time. In my A-Level History, 40% of the course was Witchcraft, 15% Cold War in Asia, 25% Modern Britain, 25% Personal Study.
By comparison, there were 8 Biology topics, all counting equally: 1- Lifestyle Health and Risk; 2- Genes and Health; 3- Voice of the Genome; 4- Biodiversity and Natural Resources; 5- On The Wild Side; 6- Immunity, Infection and Forensics; 7- Run For Your Life; 8- Grey Matter.
This is great if you like to focus on one area rather than lots, but it can get VERY boring if you (like me) don't particularly love medieval history.