This is a very odd question I know. It is said that megalithic buildings around the world are tuned to F#. What I would like to know what is the opposite of F#.

D# minor. Hope this helps!

It depends on what point of symetry you are going to take. if you take it as F then the opposite is F Flat. If you take it as C then it is F or G depending on what direction you go. If you said its A then the opposite is C. The relative minor is D# the obvious enharmonic is G flat, the relative major is A. C is on the opposite side of the circle of fifths. F is 4 keys in from the left on the keyboard and H four keys from the right so you could say the opposite is B natural (using German notation) These are the obvious definitions of opposite. Certain game shows would undoubtedly be able to define the opposite of F# to any note or even be able to say its not a note at all. If you are confused then I am by your question.

Need-a-note. In your querry the opposite note is A#. When you are speaking of Megalithic buildings I would think you need to take into account the purpose of the building. Since almost all megalithic buildings were used for spiritual and/or heavenly purposes. If you study planetary harmonics A# is complimentary and oppostie of F#.