Melbourne is technically the closest, but depending on what airline you are using it will more than likely to be cheaper to fly into MCO or WPB...
We always fly into MCO... There are more flights going into Orlando, and there are far more rental car options and possible discounts....
Melbourne is about a 40 min drive to VB.. MCO and WPB are about 90 min.. For us, the extra 50 mins in the car isn't worth the extra $$ and inconvience of having less flights to choose from.