There's a phrase that goes
Put your best foot forward. Given this phrase usually offers advice to humans who have two feet, shouldn't it be
Put your better foot forward?
Aside from the astute observation made in snailboat's comment, I still think the wording is best left as "best," because we're not really dealing with two literal feet. In the idiom, "best foot" means "best appearance," or "best first impression." In the context of, say, a job interview, putting my best foot forward means picking out my nicer clothes and shoes, making sure my hair is neatly groomed, giving a warm smile with my confident handshake, and being cordial but not arrogant. Interpreted that way, there are many more than two feet.
Very interesting question, though.
Firstly, put your best foot forward is an idiom, and hence trying to reason with it at all - never mind questioning its grammatical is likely to get you into trouble.
And secondly, if you have two of anything, then the one which is better is also the one which is best.