Ehkzu - interesting, thank you for the Alfred Deller's video I'd not heard of him. The simulation of castrato's voice in movie Farinelli was discussed in detail in complete BBC documentary "Castrato" which we were discussing here some time ago. In case you missed it then, I'll repost it here:
But the documentary felt that the movie simulation was all wrong in that they used a countertenor's voice for higher notes and female voice for the lower notes and historically they felt the other way around would've been more realistic. I actually agree with Frisco in that Maniaci's voice i.e. a voice of a male singer whose voice failed to completely break at puberty is probably closest to what castrati sounded like.
A couple of lesser known countertenors I like are Nicholas Spanos
and Jose Lemos:
Also Mathew White and a couple of others.
Of course there are also world famous ones like Andreas Scholl whose video HC posted earlier, Max Emanuel Cencic, David Daniels.