Pretty much any combination in IB would be fine since the structure of it necessarily will lead you to taking a breadth of essay writing subjects (as well as more analytical ones) and a language. Obviously taking Greek or Latin would be useful but any language background is helpful and it's quite common A-level students aren't able to study classical languages at A-level (or even at GCSE often).
Otherwise, if on offer classical civilisation as a group 3 choice would be a good option. If not, philosophy or history might be particularly relevant (as you'll probably study some philosophy and ancient history in a classics degree), although other social sciences or humanities subjects may provide useful transferable skills (and some may be more directly applicable in unusual ways, for example the recent rise of cognitive approaches to classical texts may make psychology somewhat interesting, or at least some of it).
If you are really trying to be pedantic about your choices, German or French are probably the most generally useful languages when you are studying classics as they are languages in which a lot of scholarly material has been written (i.e. papers, essays etc). But any language would be suitable to provide you experience of language learning and there will be possible benefits to most languages (although an Indo-European language might provide the best preparation, especially a synthetic language with some vestige of the IE case system e.g. German, Polish, Russian).