What would be the best GCSE's to take for the parachute regiment?

Any GCSE (or equiv) qualifications you can get prior to joining the army is great as it will open up more job roles / trades for you. Your most important are Maths and English as you will be required to gain those quals in the army if you don't gain them at GCSE level - this is due to the further quals you will gain in the army (Level 3+ quals).
My advice would be, choose the GCSE choices that you get with the ones you want to do and you think you'll do well in - if you enjoy what you have chosen you are more likely to stick at it and get a better result than if you do it because you think it's a 'better' GCSE to do for a job role or something else. English, Maths and science are important even if they are not the ones you're strongest at - try hard. Good luck.

The core subjects , Maths, English Literature, Language and Science. Maths and English will help with your career progression as you will need these in order to get promoted.