It depends on how you pronounce it. If you pronounce the middle vowel as in man, you should divide it ba-nan-a. If you pronounce the middle syllable as in spa, you should divide it ba-na-na. The first is the predominant American pronunciation. The second is the predominant British pronunciation. (And thus, the first hyphenation is the standard American one and the second is the standard British one.)
The reason the pronunciation makes a difference is the rule: never break a stressed syllable after a short vowel (i.e., /æ/, /ɛ/ /ɪ/, /ɒ/, /ʌ/, /ʊ/). There are, of course, exceptions to this rule if all the other hyphenations are worse (i.e., ra-tion).