It's generally recommended to take Higher Politics in S6 and only if you did Higher Modern Studies (or at least a higher in another social subject like History), so you'd probably be better off doing Modern Studies (even if it's a crash, you don't necessarily need any background knowledge from Nat 5).
You should definitely do Nat 5 Maths, that's a fairly "essential" qualification.
As for other subjects, are you taking a language (ie French, Spanish etc, not including English)? If you are and you're decent at it/plan to continue with it then I'd suggest that, even potentially with a crash in another language (I'm suggesting this as a possibility because I did Nat 5 French last year and am now in S5 doing Higher French and crash Higher Spanish and crashing Spanish has been surprisingly easy. I don't imagine it would be easy if I hadn't done French though obviously.)