Well, if you genuinely acted as you claim, then the offence that is alleged against you has not been committed.
What evidence exists?
On the face of it, it seems that it is a case of 'one word against the other'. In the absence of any other evidence then there is insufficient evidence to prove you guilty (beyond all reasonable doubt) of committing an assault.
What evidence exists?
If the taxi driver had a bloodied or bruised face/nose, and you have a bruised fist, you may be in trouble. More so if any forensic analysis were to show that the taxi drivers blood showed up on the sleeve of the shirt you were wearing.
Nothing else matters.
It matters not how drunk you were. The allegation against you must be proven beyond all reasonable doubt.