Well I think the best composition is an army of heavy infantry and nothing else. Heavy infantry is just strong, cheap, can be improved by ideas and laws and so be the best choice.
Such an army can be beaten by an army of mounted archers though, though you won't see such an army very often.
It may be possible, for some country to use cavalry/mounted archers only. Thoug I'm quite unsure, if an army of militia only wouldn't be much cheaper and would beat the cavalry army by masses.
I'm no big fan of combined armies, such armies like heavy infantry + archers + cavalry. I think if you concentrate your ideas and laws on heavy infantry, your heavy infantry army will simply be better than such a combined one.