Money is a good way of leverage for power. So, in my opinion, the politicians whom control the largest budget (i.e. of Europe, that would be the budget of the European parliament/European Union) are also the ones whom have the most influence on policies aka the ones in charge.

So, by that logic: in case of the country where I come from (NL) the person/party whom has the most influence would be the European Peoples Party (EPP-group) and their (albeit indirect) leader of this (federal) political group: Angela Merkel.

Merkels Party/CDU = biggest subparty of EPP and the EPP = biggest party in European Parliament. Thus, the CDU/Merkel has the most influence within the EPP and the EPP has the most influence on the budget and policies of the European Union/federation. Therefore Merkel.