I do not know for sure, but IIRC, using Opera Mini compresses data on a server first and then sends you the compressed version (almost like an image I think) which in theory greatly reduces bandwidth and increases speed. So I reckon using the mobi sites via Opera will consume the least amount of bandwidth.
That being said, using the News24, Flipboard , Tapatalk/MyBB etc apps GREATLY improves the experience and they really don't use that much bandwidth at all.
Of these Flipboard will probably use the most as it seems to download all the content of all the things you've added to it regardless of whether you open them or not (it feels like it is anyway).