What tool is best for comparing functionally identical files that may have been obfuscated?

The best tool for comparing functionally identical files that may have been obfuscated is ssdeep. This tool utilizes fuzzy hashing, which allows it to generate a hash value that can capture similarities between files even if they are not identical due to modifications like obfuscation.

When working with files that have undergone changes, traditional hashing algorithms would not be effective because even a small alteration can result in a completely different hash value. In contrast, ssdeep can identify and quantify the degree of similarity between files, making it particularly useful for analyzing potential malware or other modified files while maintaining the context of their functional equivalence.

The other options provided serve different purposes: Netcat is primarily used for network communication and doesn't focus on file comparison; Tcpdump is a network packet analyzer that captures and analyzes network traffic rather than file content; and Vmstat provides information about virtual memory statistics, which is unrelated to file comparison. Hence, ssdeep stands out as the pertinent choice for the required task of comparing obfuscated files.