Rising numbers of mobile devices and wireless access technologies motivate network operators to leverage spectrum across multiple radio access networks, in order to significantly enhance quality of service as well as network capacity. However, there is a substantial need to develop innovative network selection mechanisms that consider energy efficiency while meeting application quality requirements. In this context, this paper proposes an efficient network selection mechanism over heterogeneous wireless networks. We consider different performance aspects, as well as network characteristics and application requirements, so as to obtain an efficient solution that grasps the conflicting nature of the various objectives and addresses this ultimate tradeoff. The proposed methodology advocates a user-centric approach toward the utilization of heterogeneous wireless networks to enhance system performance and support reliable connectivity.