An Unknown Environment Penetration Test is the best approach in this scenario because it simulates the actions of an actual attacker that has no prior knowledge of the network. This type of test can provide the most realistic assessment of security as it evaluates the organization's defenses from the perspective of an uninformed attacker, which is a common threat. A Known Environment Penetration Test is not the best choice because it assumes prior knowledge of the system's internals which may not be the case for real-world attackers. The Partially Known Environment Penetration Test provides a middle ground between known and unknown and is less realistic than the unknown approach for simulating a full external threat. Reconnaissance is a phase within penetration testing rather than a complete testing approach and does not alone provide a comprehensive assessment of system vulnerabilities.