European taxes: Do we need them?

In the European Union, each member state is responsible for its tax system. Different national regimes help with tax competition, but can also lead to tax evasion or unfair rules in the Single Market. That is why better coordination or even standardisation of taxes is debated. What tax regulations are there already in the Single Market? What would be the advantages and disadvantages of a European tax? And what reforms are being discussed in Europe?