Machine learning has found its way into critical applications like medical diagnostics and credit decisions, underscoring the importance of selecting appropriate performance metrics to ensure model reliability and fairness. This guide delves into various metrics for classification, regression, forecasting, generative models, and language models, emphasizing the need for a holistic evaluation approach that goes beyond accuracy to encompass fairness, interpretability, drift resilience, and sustainability. The guide also highlights the role of platforms like Clarifai in providing tools for monitoring these metrics, supporting ethical and regulatory compliance, and facilitating model deployment in diverse environments. As AI becomes more ubiquitous, the guide stresses the importance of aligning metrics with business goals and maintaining continuous monitoring to address data drift and ensure model reliability over time.