OOOH I have some ideas!!!
the first thought in terms of unique-ness that comes to mind would def be shinji hibiki-- his pronunciation is.. wild lmao. and his voice is so cool because it is just so so WEIRD... i don't know how else to explain it? that's why i've always liked him. his creamy append is also fun to use.
Caeles is another UTAU that comes to mind as unique in the sense that there aren't many *actually* deep voiced UTAU.
and the ones that do advertise being deep are usually not god material lets be honest here. caeles' voicebank is really well made too, clear breaths, glottal stops, etc etc.
here's a good example of him in use, by his creator too!! caeles definitely stands out because his voice is deep asf and there aren't a ton of super deep-voiced UTAUs out there
and rook has not aged well.
also, i'm super basic. i know i've expressed a little bit of my unending admiration for ruko yokune's masculine libraries on here before

and while i feel like i'm gonna get shat on for calling a vipperloid "unique" i choose to defend my stance!!! bringing up their famous power (kire) bank; what is really interesting to me is how each pitch is just a little bit of a different tone from the others (i previously had a beefy paragraph explaining each but this would get too long gjshfnsdk). also, personally, i haven't used or heard a male UTAU that sports a rough vocal strain (?) effect in the higher ranges as naturally as ruko does. okay maybe there's a
little bit of bias here,
they're my favourite UTAU under merry kohaku hehe. ruko just has a really cool voice man idk!!!?! probably because Yuu Raichi (CV for ruko's masculine libraries) has such an awesome singing voice too :)))