In current industrial practice, critical path selection is an indispensable step for AC delay test and timing validation. Traditionally, this step relies on the construction of a set of worse-case paths based upon discrete timing models. The assumption of discrete timing models can be invalidated by delay effects in the deep submicron domain, where timing defects and process variation are statistical in nature. In this paper, we study the problem of optimizing critical path selection, under both fixed delay and statistical delay assumptions. With a novel problem formulation and new theoretical results, we prove that the problem in both cases are computationally intractable. We then discuss practical heuristics and their theoretical performance bounds, and demonstrate that among all heuristics under consideration, only one is theoretically feasible. Finally, we provide consistent experimental results based upon defect-injected simulation using an efficient statistical timing analysis framework.