A: No one.
Former United States Secretary of State Henry Kissenger famously asked “who do I call if I want to call Europe?”
The answer depends of course on what you want to talk about and who you want to talk to. However, Kissenger’s famous remark draws attention to the multiple layers of leadership in the European Union.
The supranational nature of the EU and its multiple institutions means that there is a “plethora of presidents” in the European Union. This is in part a question of the number of institutions and in part a question of terminology.
The European Council, the European Parliament and the European Commission all have ‘presidents’ and the Council of Ministers in addition has a rotating presidency that changes between member states every six months.
But these presidents are in effect the chairpeople of the various institutions they manage. They have influence of course (particularly in their relative institutions), but none are presidents in the same sense as the term is used in relation to the head of state in the USA or France.