Yeah, I'd pick the subject you enjoy over the subject you're best at, because you'll have more motivation to revise and work for it, and also because it may not be your best subject at A-Level- I got my best GCSE mark for Spanish, and in my January mocks it was by far my worst subject. I do History and English lit, and I've enjoyed them both, but they're both very time consuming and take a lot of work, so pick the subject you enjoy the most, because it's awful having to put effort into things you find really boring.