The 'smart city' represents a core feature of modern urban development. The appearance of numerous smart city indices, which claim to successfully evaluate and compare smart city performances, is the manifestation of the concept's growing popularity. The central objective of this paper is to address the integrity and quality of the existing smart city indices, and identify those ones which are fit for international comparison. We propose a method for evaluating the integrity and quality of the existing smart city indices in terms of five criteria \textendash Credibility, Reliability, Methodology, Versatility, Precision. The article critically evaluates six smart city indices and concludes that the Cities in Motion Index from the IESE Business School has the best overall performance. To improve the versatility of the existing smart city indices, this paper proposes a new approach based on respecting three main criteria: refining the types of indicator, defining the smart city domains and adopting context-sensitive measurements. These elements are deemed to be essential for any smart city index.