I'd also like to suggest that the question not be "most efficient" but "fastest".  I don't think it should treat "efficient" and "fast" as synonyms. 
"Efficient" can mean things other than execution speed, such as memory usage, or programmer time.  Are there space/time considerations besides execution time?  What if the user has a huge list and wants to use as little new allocated RAM as possible?

I understand that it's immediately after the "How do I speed up my program?" question, and I think it's worth considering making the question more explicit what kind of efficiency we're talking about.