Which timber of voice is considered the most rare in the opera singing?

Answer 1

If you are talking about a natural opera vocal fach or category then I believe the Dramatic Soprano and other Dramatic divisions are the most rare. In many cases the Dramatic roles are sung by people who have voices much lighter than should be used, because the existence of such voices is very rare.
If, however, you are taking of the rarity of availability... some male voices may be harder to come by on an every-day basis.
If you are talking about quality within a division... the amount of truly historic voices has dropped since the wave of Wonders that sang their prime through the 50's, 60's, and 70's. I feel that is not so very much the lack of voices, but of good technique and work.

Answer 2

The rarest these days is the male soprano (not the counter-tenor) In the days when they were popular, the Male Soprano had a major role in music, but due to the PHYSICAL constraints, (castration) they became less and less, until today there are none.

Answer 3

Either the counter-tenor or something like Spinto Soprano: A full lyric voice that can be pushed to dramatic climaxes. I have been told that with the right training, I could become a spinto. (currently a mezzo. woot.

Answer 4

That's a hard one. I don't think I would even venture an opinion other than to say, that I've been told that colorotura sopranos (that's me!) are harder to come by than other kinds.